70 THE FACTORS OF ORGANIC EVOLUTION. 



individual in which they were a little more deeply seated 

 would be less likely to be incapacitated by injury of them. 

 And so in multitudinous other ways. But nevertheless, as 

 we here see, natural selection could operate only under 

 subjection. It could do no more than take advantage of 

 those structural changes which the medium and its con 

 tents initiated. 



See, then, how large has been the part played by this 

 primordial factor. Had it done no more than give to 

 Protozoa and Protopliyta that cell-form which characterizes 

 them had it done no more than entail the cellular com 

 position which is so remarkable a trait of Metazoa and 

 Metaphyta had it done no more than cause the repetition 

 in all visible animals and plants of that primary differen 

 tiation of outer from inner which it first wrought in 

 animals and plants invisible to the naked eye; it would 

 have done much towards giving to organisms of all kinds 

 certain leading traits. But it has done more than this. 

 By causing the first differentiations of those clusters of 

 units out of which visible animals in general arose, it 

 fixed the starting place for organization, and therefore 

 determined the course of organization; and, doing this, gave 

 indelible traits to embryonic transformations and to adult 

 structures. 



Though mainly carried on after the inductive method, the 

 argument at the close of the foregoing section has passed 

 into the deductive. Here let us follow for a space the 

 deductive method pure and simple. Doubtless in biology 

 a priori reasoning is dangerous; but there can be no 

 danger in considering whether its results coincide with 

 those reached by reasoning a posteriori. 



Biologists in general agree that in the present state of 

 the world, no such thing happens as the rise of a living 

 creature out of non-living matter. They do not deny, 

 however, that at a remote period in the past, when the 



