172 NATURAL SELECTION Vill 
ditions in which it is placed. But it will now be a different 
creature. It will be not only swifter and stronger, and more 
furry—it will also probably have changed in colour, in form, 
perhaps have acquired a longer tail, or differently shaped 
ears ; for it is an ascertained fact that when one part of an 
animal is modified, some other parts almost always change, 
as it were in sympathy with it. Mr. Darwin calls this 
“correlation of growth,” and gives as instances that hairless 
dogs have imperfect teeth; white cats, when blue-eyed, are 
deaf ; small feet accompany short beaks in pigeons ; and other 
equally interesting cases. 
Grant, therefore, the premises: 1st, That peculiarities of 
every kind are more or less hereditary; 2d, That the off- 
spring of every animal vary more or less in all parts of their 
organisation ; 3d, That the universe in which these animals 
live is not absolutely invariable ;—none of which proposi- 
tions can be denied ; and then consider that the animals in 
any country (those at least which are not dying out) must at 
each successive period be brought into harmony with the 
surrounding conditions ; and we have all the elements for a 
change of form and structure in the animals, keeping exact 
pace with changes of whatever nature in the surrounding 
universe. Such changes must be slow, for the changes in the 
universe are very slow; but just as these slow changes be- 
come important, when we look at results after long periods 
of action,—as we do when we perceive the alterations of the 
earth’s surface during geological epochs, —so the parallel 
changes in animal form become more and more striking, in 
proportion as the time they have been going on is great; as 
we see when we compare our living animals with those 
which we disentomb from each successively older geological 
formation. 
This is, briefly, the theory of natural selection, which 
explains the changes in the organic world as being parallel 
with, and in part dependent on, those in the inorganic. What 
we now have to inquire is, Can this theory be applied in 
any way to the question of the origin of the races of man ? or 
is there anything in human nature that takes him out of the 
category of those organic existences over whose successive 
mutations it has had such powerful sway ? 
