2o6 LAMARCK, HIS LIFE AND WORK 



that is to say, become perfected or degraded, through 

 great changes in the distribution of land and ocean ; 

 througli the cultivation or neglect of the countiy 

 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." * 



The following passages, debarring the error of deriv- 

 ing 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 con- 

 tribution to the theory of descent. 



" It is probable, then, that all the animals of the 

 New World are derived from congeners in the Old, 

 without any deviation from the ordinary course of 

 nature. We may believe that, having become sepa- 

 rated in the lapse of ages by vast oceans and countries 

 which they could not traverse, they have gradually 

 been affected by, and derived impressions from, a 

 climate which has itself been modified so as to be- 

 come a new one through the operations of those same 

 causes which dissociated the individuals of the Old and 

 the New World from one another; thus in the course 

 of time they have grown smaller and changed their 



*Tome ix., p. 127, 1761 (ex. Butler). 



