"[W] hile providing no definite quantitative or qualitative tests by which enforcement agencies could gauge the effects of a given merger to determine whether it may "substantially" lessen competition or tend toward monopoly, Congress indicated plainly that a merger had to be functionally viewed, in the context of its particular industry. That is, whether the consolidation was to take place in an industry that was fragmented rather than concentrated, that had seen a recent trend toward domination by a few leaders or had remained fairly consistent in its distribution of market shares among the participating companies, that had experienced easy access to markets by suppliers and easy access to suppliers by buyers or had witnessed foreclosure of business, that had witnessed the ready entry to new competition or the erection of barriers to prospective entrants, all were aspects, varying in importance with the merger under consideration, which would properly be taken into account.

Eighth, Congress used the words, "may be substantially to lessen competition" (emphasis supplied), to indicate that its concern was with probabilities, not certainties. Statues existed for dealing with clear-cut menaces to competition; no statute was sought for dealing with ephemeral possibilities. Mergers with a probable anticompetitive effect were to be proscribed by this Act."

Chief Justice Earl Warren. Brown Shoe v. U. S. 370 U.S. 294 (1962)