Under the head of geographical distribution (in tome ix., 1761), in which subject Buffon made his most original contribution to exact biology, he claims to have been the first "even to have suspected" that not a single tropical species is common to both eastern and western continents, but that the animals common to both continents are those adapted to a temperate or cold climate. He even anticipates the subject of migration in past geological times by supposing that those forms travelled from the Old World either over some land still unknown, or "more probably" over territory which has long since been submerged.[135] The mammoth "was certainly the greatest and strongest of all quadrupeds, but it has disappeared; and if so, how many smaller, feebler, and less remarkable species must have perished without leaving us any traces or even hints of their having existed? How many other species have changed their nature, that is to say, become perfected or degraded, through great changes in the distribution of land and ocean; through the cultivation or neglect of the country which they inhabit; through the long-continued effects of climatic changes, so that they are no longer the same animals that they once were. Yet of all living beings after man the quadrupeds are the ones whose nature is most fixed and form most constant; birds and fishes vary much more easily; insects still more again than these; and if we descend to plants, which certainly cannot be excluded from animated nature, we shall be surprised at the readiness with which species are seen to vary, and at the ease with which they change their forms and adopt new natures."[136] The following passages, debarring the error of deriving all the American from the Old World forms, and the mistake in supposing that the American forms grew smaller than their ancestors in the Old World, certainly smack of the principle of isolation and segregation, and this is Buffon's most important contribution to the theory of descent.