Charles Darwin

Charles Robert Darwin was the fifth child of Robert Waring Darwin, a physician, and Susannah Wedgewood, from a well to-do family.   He attended Shrewsbury Grammar School.  From early on, he was an amateur collector of natural products and coins.  In 1825, at the age of sixteen, he entered the University of Edinburgh to study medicine, where he found dread the screams of patients while being operated without anesthesia.  There he was influenced by the Lamarckian Robert Edmund Grant.  After two years he quit school and went to live with his Uncle Josiah Wedgewood.  He then entered Cambridge University to study to be a clergyman, where he met John Steven Henslow who helped him regain his interest in nature.  Henslow would later be the key figure getting Darwin the position of naturalist on the The Beagle despite his father's opposition.

On 2 October 1836 he embarked in H.M.S. Beagle, under the command of Captain Fitzroy, with the title of naturalist, an unpaid position.  The trip lasted five years and Darwin visited, among other places, the Cape Verde Archipelago, the Falkland Islands, the South American coast, the Galapagos Islands and Australia, collecting considerable quantities of specimens.

After returning from the voyage in 1836, he started analyzing the specimens and notes he took.  He noticed noticed the similarities between fossils and living species within the same geographic area.  In the Galapagos Islands, he noted that every island had its own kind of tortoises and birds that were all slightly different in appearance related to the kind of food they ate.  As a result of that he started writing what he called Notebook on the Transmutation of Species, a book greatly influenced by Thomas Malthus' Essay on the Principle of Population and Lyell's Principles of Geology.

He began writing his synthetic work on the origin of species in 1842.  In the meantime he married his cousin Emma Wedgwood in 1839.  After living for a number of years in London he moved to Down House, in Downe, Kent.  Darwin and his wife had ten children, three of whom died early.  Between 1839 and 1843 he published the five volumes of Zoology of the Voyage of H.M.S. Beagle.  After receiving a letter from Alfred Russell Wallace, another British naturalist, containing essential the same ideas on evolution by natural selection (but in a more much abbreviated form), Darwin's colleagues convinced him to present a joint Darwin-Wallace paper before the Linnean Society in London on 1 July 1858, tiled The Origin of Species by Means of Natural Selection.

Darwin's book On the Origin of Species by Means of Natural Selection was published in 1859 and all copies were sold the first day of publication.  That book was followed by five other editions (see below) as well as by the following titles: The Variation of Animals and Plants Under Domestication (1868), The Descent of Man, and Selection in Relation to Sex (1871) and The Expression of Emotions in Animals and Man (1872).

Darwin died of a heart attack and was buried in Westminster Abbey.

He is considered the father of modern evolutionary thought.

Darwin's Influence on Evolutionary and Ecological Ideas on Cave Biology

Although Charles Darwin never studied cave fauna first hand, he was interested in the topic, particularly related to three issues: a) cave colonization, b) affinities between the cave fauna and its presumed ancestors in the surrounding areas, and, c) rudimentation or the loss of organs, particularly eyes and what he saw as a compensatory process, i.e., the enlargement of other sensory organs.

The first time that Darwin seems to be mingling about this issue was 8 December 1844 where in the entries of his notebook about a conversation he had with J. Hooker he wrote that 'our cow , which has two abortive mammae, then these two are uniquely developed' adding later 'Believe part, which is normally in a species abortive appears often as a rudiment- Has lately seen and describe this in case of pistil of dioecious Umbellieferous plant: does not know anything on Bentham's law of variability of abortive parts.' (Burkhardt and Smith 1987, pp. 400-403).

In a 8 May 1852 letter to James Dwight Dana (Burkhardt and Smith 1989, p. 92), he shows great deal of interest on the cave fauna of Mammoth Cave as described by Silliman (1851) and asks if he could receive a specimen of the blind rat which he would like to forward to the British Museum.  Silliman in his paper mentions several species of blind and depigmented animals including fish, insects and crustaceans.  Some of them appear to have elongated antennae.  Special mention is made of the cave rat, with large but apparently non-functional eyes which, according to Silliman, 'By keeping them however in captivity and diffuse light they gradually appeared to attain some power of vision.'  Darwin will cite all these facts in his Origin (pp. 137-138), speculating that 'in the case of the cave-rat natural selection seems to have struggled with the loss of light and to have increased the size of the eyes; whereas with all the other inhabitants of the caves, disuse by itself seems to have done its work.'

Darwin received a letter dated 23 November 1856 from John Obadiah Westwood (1805-1893), a British entomologist, giving him some taxonomical and distributional information about cave insects from both North America and Europe (Burkhardt and Smith 1990, p. 283-284).

Darwin received another letter from Dana dated 8 December 1856 in which the American naturalist tells Darwin that he had confirmed with Agassiz that the blind rat of the Mammoth Cave is 'American in type.'  Then Dana goes into a long disquisition about the idea of progress 'a law which involves the expression of a type-idea in forms or groups of increasing diversity, and generally of higher elevation; always resulting in a purer & fuller exhibition of the type' and that 'it is the simple before the complex' (Burkhardt and Smith 1990, p. 299-300).

On 14 July 1856 he wrote James Dwight Dana asking if he could get more anatomical information about North American cave fauna, particularly arthropods (Burkhardt and Smith 1990, p. 180).  Dana replies to him with mostly systematic information on 8 September 1856 (Burkhardt and Smith 1990, pp. 215-217).  On 29 September 1856, Darwin replies Dana thanking him for the information and asking for addition information on the blind rat (Burkhardt and Smith 1990, p. 235-237).

Therefore, Darwin wrote on cave fauna based on the observations made by the Americans Benjamin Silliman, Jr. (1816-1885) and James Dwight Dana (1813-1895, who was Silliman's brother-in-law) and the Danish naturalist Jorgen Matthias Christian Schiödte (1815-1884). 

He noted that cave fauna was more closely related to the fauna of the surrounding regions than elsewhere, as is the case for fauna of other isolated habitats.  Thus, he argued that the cave fauna descended from the fauna of the surrounding region, ‘the colonists having been subsequently modified and better fitted to their new homes’ (Darwin 1859, p. 403).  At first Darwin considered the mechanisms of both natural selection and disuse to explain troglomorphic features, i.e., enlargement of some sensory systems and appendages for the former; blindness and depigmentation for the latter.  To Darwin, thus, suggested a ‘contest ... between selection enlarging and disuse alone reducing these organs’ (Darwin 1859, p. 296).  However, by the third edition of the Origin (Darwin 1861) he de-emphasized the importance of natural selection, eliminating the speculation of a ‘contest’ between selection and disuse.  In fact, in the first two editions, in the paragraphs relative to cave animals and rudimentation, he used the words disuse and selection seven times each; by the third edition, it was five and two, respectively.

James Dwight Dana	Benjamin Silliman, Jr.

Which one is the real Darwin when it comes to the evolution of cave faunas and the phenomenon of rudimentation?  Essentially he was a Lamarckian when it came to the evolution of cave fauna; therefore, to think that later neo-Lamarckism was 'anti-Darwinian' is a misinterpretation of the facts, since Darwin himself held those ideas.  In many ways, Darwin maintained a modified version of the Great Chain of Being (Bowler 1984, pp. 55-59), which was championed by the Swiss naturalist Charles Bonnet (1720-1793) and the French Philosopher Jean-Baptiste Robinet (1735-1820).  They were strict followers of Jean Baptiste Chevalier de Lamarck (b. Bazentin-le-Petit, Picardy, France, 1 August 1744; d. Paris, France, 28 December 1829).  Lamarck argued that organisms experience ‘needs’ (besoins) which were brought about by the environment and triggered fluids (including electricity) which, when circulated in the body, enlarged or developed the appropriate organ.  In ‘higher’ animals, a crucial causal factor was the ‘inner consciousness’ (sentiment interieur), which makes parts respond and develop.  This resulted in the inheritance of derived characters.  Even long after the development of the Modern Synthesis, this philosophy and its jargon (e.g., ‘regressive evolution’) would continue to be strongly embedded in biospeleological research.

Charles Bonnet	Jean Baptiste Chevalier de Lamarck

A Matter of Editions

Although Darwin's original intention was to make a few corrections, it clearly represents Darwin's responses to all the criticisms (both private and public) he received from the first edition.  The third (1861), fourth (1866), fifth (1869), and sixth (1872) editions do contain major corrections in which Darwin tries to assimilate the responses and critiques of his contemporaries.

Darwin's text on cave animals in his first edition of the Origin of Species (1859).  With the exception of the word 'easily' (see it in bold), which was excised in the second edition, both texts are identical

It is well known that several animals, belonging to the most different classes, which inhabit the caves of Styria and of Kentucky, are blind.

In some of the crabs the foot-stalk for the eye remains, though the

eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost.  As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse.  In one of the blind animals, namely, the cave-rat, the eyes are of immense size; and Professor Silliman thought that it regained, after living some days in the light, some slight power of vision.  In the same manner as in Madeira the wings of some of the insects have been enlarged, and the wings of others have been reduced by natural selection aided by use and disuse, so in the case of the cave-rat natural selection seems to have struggled with the loss of light and to have increased the size of the eyes; whereas with all the other inhabitants of the caves, disuse by itself seems to have done its work.

It is well known that several animals, belonging to the most different classes, which inhabit the caves of Carniola and Kentucky, are blind. In some of the crabs the foot-stalk for the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, their loss may be attributed to disuse.  In one of the blind animals, namely, the cave-rat (Neotoma), two of which were captured by Professor Silliman at above half a mile distance from the mouth of the cave, and therefore not in the profoundest depths, the eyes were lustrous and of large size; and these animals, as I am informed by Professor Silliman, after having been exposed for about a month to a graduated light, acquired a dim perception of objects.

(Notice that here Darwin drops the idea of the 'struggle' between natural selection and disuse; selection is not even mentioned)

(...)

(In contrast to the first two editions, Darwin expands in terms of examples and argument his contention that cave animals derived from surface ones in the adjacent areas; however, the most important new point made in the third edition is the introduction of the idea of preadaptation, although not using the term, when saying that 'it is natural that an insect already deprived of vision should readily become adapted to dark caverns.')

(…)

The principle which determines the general character of the fauna and flora of oceanic islands, namely, that the inhabitants, when not identically the same, yet are plainly related to the inhabitants of that region whence colonists could most readily have been derived,--the colonists having been subsequently modified and better fitted to their new homes,--is of the widest application throughout nature.  We see this on every mountain, in every lake and marsh.  For Alpine species, excepting in so far as the same forms, chiefly of plants, have spread widely throughout the world during the recent Glacial epoch, are related to those of the surrounding lowlands;--thus we have in South America, Alpine humming-birds, Alpine rodents, Alpine plants, &c., all of strictly American forms, and it is obvious that a mountain, as it became slowly upheaved, would naturally be colonised from the surrounding lowlands. So it is with the inhabitants of lakes and marshes, excepting in so far as great facility of transport has given the same general forms to the whole world.  We see this same principle in the blind animals inhabiting the caves of America and of Europe.  Other analogous facts could be given.  And it will, I believe, be universally found to be true, that wherever in two regions, let them be ever so distant, many closely allied or representative species occur, there will likewise be found some identical species, showing, in accordance with the foregoing view, that at some former period there has been intercommunication or migration between the two regions.  And wherever many closely-allied species occur, there will be found many forms which some naturalists rank as distinct species, and some as varieties; these doubtful forms showing us the steps in the process of modification.

(...)

 The same principle which governs the general character of the inhabitants of oceanic islands, namely, the relation to the source whence colonists could have been most easily derived, together with their subsequent modification, is of the widest application throughout nature. We see this on every mountain-summit, in every lake and marsh. For Alpine species, excepting in as far as the same species have become widely spread during the Glacial epoch, are related to those of the surrounding lowlands; thus we have in South America, Alpine humming-birds, Alpine rodents, Alpine plants, etc., all strictly belonging to American forms; and it is obvious that a mountain, as it became slowly upheaved, would be colonised from the surrounding lowlands. So it is with the inhabitants of lakes and marshes, excepting in so far as great facility of transport has allowed the same forms to prevail throughout large portions of the world. We see the same principle in the character of most of the blind animals inhabiting the caves of America and of Europe. Other analogous facts could be given. It will, I believe, be found universally true, that wherever in two regions, let them be ever so distant, many closely allied or representative species occur, there will likewise be found some identical species; and wherever many closely-allied species occur, there will be found many forms which some naturalists rank as distinct species, and others as mere varieties; these doubtful forms showing us the steps in the process of modification.

(The differences here are essentially of wording)

(…)

Any change in function, which can be effected by insensibly small steps, is within the power of natural selection; so that an organ rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose.  Or an organ might be retained for one alone of its former functions.  An organ, when rendered useless, may well be variable, for its variations cannot be checked by natural selection.  At whatever period of life disuse or selection reduces an organ, and this will generally be when the being has come to maturity and to its full powers of action, the principle of inheritance at corresponding ages will reproduce the organ in its reduced state at the same age, and consequently will seldom affect or reduce it in the embryo.  Thus we can understand the greater relative size of rudimentary organs in the embryo, and their lesser relative size in the adult.  But if each step of the process of reduction were to be inherited, not at the corresponding age, but at an extremely early period of life (as we have good reason to believe to be possible) the rudimentary part would tend to be wholly lost, and we should have a case of complete abortion.  The principle, also, of economy, explained in a former chapter, by which the materials forming any part or structure, if not useful to the possessor, will be saved as far as is possible, will probably often come into play; and this will tend to cause the entire obliteration of a rudimentary organ.

As the presence of rudimentary organs is thus due to the tendency in every part of the organisation, which has long existed, to be inherited--we can understand, on the genealogical view of classification, how it is that systematists have found rudimentary parts as useful as, or even sometimes more useful than, parts of high physiological importance.  Rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue in seeking for its derivation.  On the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the ordinary doctrine of creation, might even have been anticipated, and can be accounted for by the laws of inheritance.

(...)

It appears probable that disuse has been the main agent in rendering organs rudimentary. It would at first lead by slow steps to the more and more complete reduction of a part, until at last it became rudimentary--as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced by beasts of prey to take flight, and have ultimately lost the power of flying. Again, an organ, useful under certain conditions, might become injurious under others, as with the wings of beetles living on small and exposed islands; and in this case natural selection will have aided in reducing the organ, until it was rendered harmless and rudimentary.

(Once again Darwin drops any reference to selection while streamlining his argument in favor of disuse) 

(…)

The complex and little known laws governing the production of varieties are the same, as far as we can judge, with the laws which have governed the production of distinct species. In both cases physical conditions seem to have produced some direct and definite effect, but how much we cannot say. Thus, when varieties enter any new station, they occasionally assume some of the characters proper to the species of that station. With both varieties and species, use and disuse seem to have produced a considerable effect; for it is impossible to resist this conclusion when we look, for instance, at the logger-headed duck, which has wings incapable of flight, in nearly the same condition as in the domestic duck; or when we look at the burrowing tucu-tucu, which is occasionally blind, and then at certain moles, which are habitually blind and have their eyes covered with skin; or when we look at the blind animals inhabiting the dark caves of America and Europe. With varieties and species, correlated variation seems to have played an important part, so that when one part has been modified other parts have been necessarily modified. With both varieties and species, reversions to long-lost characters occasionally occur. How inexplicable on the theory of creation is the occasional appearance of stripes on the shoulders and legs of the several species of the horse-genus and of their hybrids! How simply is this fact explained if we believe that these species are all descended from a striped progenitor, in the same manner as the several domestic breeds of the pigeon are descended from the blue and barred rock-pigeon!

(The differences here are essentially a matter of wording)

The Sixth Edition

By the sixth edition of the Origin, Darwin got more sophisticated, tried to add some evidence to support his statements but remain cautious about the role that natural selection may have played in the reduction of phenotypic characters, by saying that this process may have been 'aided perhaps by natural selection.'  By now the number of times he uses the word 'disuse' (when discussing cave organisms and rudimentation) has risen to nine, but 'selection' is even higher: ten.  Also, he added the idea that animals subjected to darkness may develop 'inflammations of the eyes' and that the covering of those organs by tissue ca 'be an advantage' (that is where selection may play a role).  He then goes into the statement that rudimentary organs are very common among many organisms (a true fact usually overlook by the practitioners of the regressive 'evolution concept' today).  He does so by providing numerous examples from plants to whales.  Yet, he remains firmed on his idea that 'It appears probable that disuse has been the main agent in rendering organs rudimentary.'  Although he points out to the benefit for an organism to reduce organs that are no longer utilized for the sake of 'economy', he acknowledges that he has no explanation on why total disappearance of those organs occur.

The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. In South America, a burrowing rodent, the tucutucu, or Ctenomys, is even more subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they were frequently blind.

One which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. As frequent inflammations of the eyes must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would aid the effects of disuse.

It is well known that several animals, belonging to the most different classes, which inhabit the caves of Carniola and of Kentucky, are blind.

In some of the crabs the footstalk for the eye remains, though the eye is gone;—the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, their loss may be attributed to disuse. In one of the blind animals, namely, the cave rat (Neotoma), two of which were captured by Professor Silliman at above half a mile distance from the mouth of the cave, and therefore not in the profoundest depths, the eyes were lustrous and of large size; and these animals, as I am informed by Professor Silliman, after having been exposed for about a month to a graduated light, acquired a dim perception of objects.

It is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar climate; so that, in accordance with the old view of the blind animals having been separately created for the American and European caverns, very close similarity in their organisation and affinities might have been expected. This is certainly not the case if we look at the two whole faunas; and with respect to the insects alone, Schiödte has remarked “We are accordingly prevented from considering the entire phenomenon in any other light than something purely local, and the similarity which is exhibited in a few forms between the Mammoth cave (in Kentucky) and the caves in Carniola, otherwise than as a very plain expression of that analogy which subsists

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generally between the fauna of Europe and of North America.” On my view we must suppose that American animals, having in most cases ordinary powers of vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves, as did European animals into the caves of Europe. We have some evidence of this gradation of habit; for, as Schiödte remarks, “We accordingly look upon the subterranean faunas as small ramifications which have penetrated into the earth from the geographically limited faunas of the adjacent tracts, and which, as they extended themselves into darkness, have been accommodated to surrounding circumstances. Animals not far remote from ordinary forms, prepare the transition from light to darkness. Next follow those that are constructed for twilight; and, last of all, those destined for total darkness, and whose formation is quite peculiar.” These remarks of Schiödte’s, it should be understood, apply not to the same, but to distinct species. By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have effected other changes, such as an increase in the length of the antennæ or palpi, as a compensation for blindness. Notwithstanding such modifications, we might expect still to see in the cave-animals of America, affinities to the other inhabitants of that continent, and in those of Europe to the inhabitants of the European continent. And this is the case with some of the American cave-animals, as I hear from Professor Dana; and some of the European cave-insects are very closely allied to those of the surrounding country. It would be difficult to give any rational explanation of the affinities of the blind cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation. That several of the inhabitants of the caves of the Old and New Worlds should be closely related, we might expect from the well-known relationship of most of their other productions. As a blind species of Bathyscia is found in abundance on shady rocks far from caves, the loss of vision in the cave-species of this one genus has probably had no relation to its dark habitation; for it is natural that an insect already deprived of vision should readily become adapted to dark caverns. Another blind genus (Anophthalmus) offers this remarkable peculiarity, that the species, as Mr. Murray observes, have not as yet been found anywhere except in caves, yet those which inhabit the several caves of Europe and America are distinct; but it is possible that the progenitors of these several species, whilst they were furnished with eyes, may formerly

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have ranged over both continents, and then have become extinct, excepting in their present secluded abodes. Far from feeling surprise that some of the cave animals should be very anomalous, as from Agassiz has remarked in regard to the blind fish, the Amblyopsis, and as is the case with the blind Proteus with reference to the reptiles of Europe, I am only surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the scanty inhabitants of these dark abodes will have been exposed. 

Rudimentary, Atrophied, and Aborted Organs.

Organs or parts in this strange condition, bearing the plain stamp of inutility, are extremely common, or even general, throughout nature. It would be impossible to name one of the higher animals in which some part or other is not in a rudimentary condition. In the mammalia, for instance, the males possess rudimentary mammæ; in snakes one lobe of the lungs is rudimentary; in birds the “bastard-wing” may safely be considered as a rudimentary digit, and in some species the whole wing is so far rudimentary that it cannot be used for flight. What can be more curious than the presence of teeth in foetal whales, which when grown up have not a tooth in their heads; or the teeth, which never cut through the gums, in the upper jaws of unborn calves?

Rudimentary organs plainly declare their origin and meaning in various ways. There are beetles belonging to closely allied species, or even to the same identical species, which have either full-sized and perfect wings, or mere rudiments of membrane, which not rarely lie under wing-covers firmly soldered together; and in these cases it is impossible to doubt, that the rudiments represent wings. Rudimentary organs sometimes retain their potentiality: this occasionally occurs with the mammae of male mammals, which have been known to become well developed and to secrete milk. So again in the udders in the genus Bos, there are normally four developed and two rudimentary teats; but the latter in our domestic cows sometimes become well developed and yield milk. In regard to plants the petals are sometimes rudimentary, and sometimes well-developed in the individuals of the same species. In certain plants having separated sexes Kölreuter found that by crossing a species, in which the male flowers included a rudiment of a pistil, with an hermaphrodite species, having of course a well-developed pistil, the rudiment in the hybrid offspring was much increased in size; and this clearly shows that the rudimentary and perfect pistils are essentially alike in nature. An animal may posses various parts in a perfect state, and yet they may in one sense be rudimentary, for they are useless: thus the tadpole of the common salamander or water newt, as Mr. G. H. Lewes remarks, “has gills, and passes it existence in the water; but the Salamandra atra, which lives high up among the mountains, brings forth its young full-formed. This animal never lives in the water. Yet if we open a gravid female, we find tadpoles inside her with exquisitely feathered gills; and when placed in water they swim about like the tadpoles of the water-newt. Obviously this aquatic organisation has

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no reference to the future life of the animal, nor has it any adaptation to its embryonic condition; it has solely reference to ancestral adaptations, it repeats a phase in the development of its progenitors.”

An organ, serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose, and remain perfectly efficient for the other. Thus, in plants, the office of the pistil is to allow the pollen-tubes to reach the ovules within the ovarium. The pistil consists of a stigma supported on a style; but in some Compositæ, the male florets, which of course cannot be fecundated, have a rudimentary pistil, for it is not crowned with a stigma; but the style remains well developed and is clothed in the usual manner with hairs, which serve to brush the pollen out of the surrounding and conjoined anthers. Again, an organ may become rudimentary for its proper purpose, and be used for a distinct one: in certain fishes the swim bladder seems to be rudimentary for its proper function of giving buoyancy, but has become converted into a nascent breathing organ or lung. Many similar instances could be given.

Useful organs, however little they may be developed, unless we have reason to suppose that they were formerly more highly developed, ought not to be considered as rudimentary. They may be in a nascent condition, and in progress towards further development. Rudimentary organs, on the other hand, are either quite useless, such as teeth which never cut through the gums, or almost useless, such as the wings of an ostrich, which serve merely as sails. As organs in this condition would formerly, when still less developed, have been of even less use than at present, they cannot formerly have been produced through variation and natural selection, which acts solely by the preservation of useful modifications. They have been partially retained by the power of inheritance, and relate to a former state of things. It is, however, often difficult to distinguish between rudimentary and nascent organs; for we can judge only by analogy whether a part is capable of further development, in which case alone it deserves to be called nascent. Organs in this condition will always be somewhat rare; for beings thus provided will commonly have been supplanted by their successors with the same organ in a more perfect state, and consequently will have become long ago extinct. The wing of the penguin is of high service, acting as a fin; it may, therefore, represent the nascent state of the wing; not that I believe this to be the case; it is more probably a reduced organ, modified for a new function; the wing of the Apteryx, on the other hand, is quite

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useless, and is truly rudimentary. Owen considers the simple filamentary limbs of the Lepidosiren as the “beginnings of organs which attain full functional development in higher vertebrates”; but, according to the view lately advocated by Dr. Günther, they are probably remnants, consisting of the persistent axis of a fin, with the lateral rays or branches aborted. The mammary glans of the Ornithorhynchus may be considered, in comparison with the udders of a cow, as in a nascent condition. The ovigerous frena of certain cirripedes, which have ceased to give attachment to the ova and are feebly developed, are nascent branchiæ.

It is an important fact that rudimentary organs, such as teeth in the upper jaws of whales and ruminants, can often be detected in the embryo, but afterwards wholly disappear. It is also, I believe, a universal rule, that a rudimentary part is of greater size in the embryo relatively to the adjoining parts, than in the adult; so that the organ at this early age is less rudimentary, or even cannot be said to be in any degree rudimentary. Hence rudimentary organs in the adult are often said to have retained their embryonic condition.

I have now given the leading facts with respect to rudimentary organs. In reflecting on them, every one must be struck with astonishment; for the same reasoning power which tells us that most parts and organs are exquisitely adapted for certain purposes, tells us with equal plainness that these rudimentary or atrophied

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organs are imperfect and useless. In works on natural history, rudimentary organs are generally said to have been created “for the sake of symmetry,” or in order “to complete the scheme of nature.” But this is not an explanation, merely a re-statement of the fact. Nor is it consistent with itself: thus the boa-constrictor has rudiments of hindlimbs and of a pelvis, and if it be said that these bones have been retained “to complete the scheme of nature,” why, as Professor Weismann asks, have they not been retained by other snakes, which do not possess even a vestige of these same bones? What would be thought of an astronomer who maintained that the satellites revolve in elliptic courses round their planets “for the sake of symmetry”; because the planets thus revolve round the sun? An eminent physiologist accounts for the presence of rudimentary organs, by supposing that they serve to excrete matter in excess, or matter injurious to the system; but can we suppose that the minute papilla, which often represents the pistil in male flowers, and which is formed of mere cellular tissue, can thus act? Can we suppose that rudimentary teeth, which are subsequently absorbed, are beneficial to the rapidly growing embryonic calf by removing matter so precious as phosphate of lime? When a man’s fingers have been amputated, imperfect nails have been known to appear on the stumps, and I could as soon believe that these vestiges of nails are developed in order to excrete horny matter, as that the rudimentary nails on the fin of the manatee have been developed for this same purpose.

On the view of descent with modification, the origin of rudimentary organs is comparatively simple; and we can understand to a large extent the laws governing their imperfect development. We have plenty of cases of rudimentary organs in our domestic productions,—as the stump of a tail in tailless breeds,—the vestige of an ear in earless breeds of sheep, the reappearance of minute dangling horns in hornless breeds of cattle, more especially, according to Youatt, in young animals,—and the state of the whole flower in the cauliflower. We often see rudiments of various parts in monsters; but I doubt whether any of these cases throw light on the origin of rudimentary organs in a state of nature, further than by showing that rudiments can be produced; for the balance of evidence clearly indicates that species under nature do not undergo great and abrupt changes. But we learn from the study of our domestic productions that the disuse of parts leads to their reduced size; and that the result is inherited.

It appears probable that disuse has been the main agent in

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rendering organs rudimentary. It would at first lead by slow steps to the more and more complete reduction of a part, until at last it became rudimentary,—as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced by beasts of prey to take flight, and have ultimately lost the power of flying. Again, an organ, useful under certain conditions, might become injurious under others, as with the wings of beetles living on small and exposed islands; and in this case natural selection will have aided in reducing the organ, until it was rendered harmless and rudimentary.

Any change in structure and function, which can be effected by small stages, is within the power of natural selection; so that an organ rendered, through changed habits of life, useless or injurious for one purpose, might be modified and used for another purpose. An organ might, also, be retained for one alone of its former functions. Organs, originally formed by the aid of natural selection, when rendered useless may well be variable, for their variations can no longer be checked by natural selection. All this agrees well with what we see under nature. Moreover, at whatever period of life either disuse or selection reduces an organ, and this will generally be when the being has come to maturity and has to exert its full powers of action, the principle of inheritance at corresponding ages will tend to reproduce the organ in its reduced state at the same mature age, but will seldom affect it in the embryo. Thus we can understand the greater size of rudimentary organs in the embryo relatively to the adjoining parts, and their lesser relative size in the adult. If, for instance, the digit of an adult animal was used less and less during many generations, owing to some change of habits, or if an organ or gland was less and less functionally exercised, we may infer that it would become reduced in size in the adult descendants of this animal, but would retain nearly its original standard of development in the embryo.

There remains, however, this difficulty. After an organ has ceased being used, and has become in consequence much reduced, how can it be still further reduced in size until the merest vestige is left; and how can it be finally quite obliterated? It is scarcely possible that disuse can go on producing any further effect after the organ has once been rendered functionless. Some additional explanation is here requisite which I cannot give. If, for instance, it could be proved that every part of the organisation tends to vary in a greater degree towards diminution than towards augmentation of size, then we should be able to understand how an organ which has become useless would be rendered, independently of the

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effects of disuse, rudimentary, and would at last be wholly suppressed; for the variations towards diminished size would no longer be checked by natural selection. The principle of the economy of growth, explained in a former chapter, by which the materials forming any part, if not useful to the possessor, are saved as far as is possible, will perhaps come into play in rendering a useless part rudimentary. But this principle will almost necessarily be confined to the earlier stages of the process of reduction; for we cannot suppose that a minute papilla, for instance, representing in a male flower the pistil of the female flower, and formed merely of cellular tissue, could be further reduced or absorbed for the sake of economising nutriment.

Finally, as rudimentary organs, by whatever steps they may have been degraded into their present useless condition, are the record of a former state of things, and have been retained solely through the power of inheritance,—we can understand, on the genealogical view of classification, how it is that systematists, in placing organisms in their proper places in the natural system, have often found rudimentary parts as useful as, or even sometimes more useful than, parts of high physiological importance. Rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue for its derivation. On the view of descent with modification, we may conclude that the existence of organs in rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the old doctrine of creation, might even have been anticipated in accordance with the views here explained.