206 BAMAR CIE 11S LiL, ALD VO Reba 
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 
aS 
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 
MVOMe IxX., p.1T27, D7Ol (ere butler), 
