THE FACTORS OF ORGANIC EVOLUTION. 201 



face was the part through which nutritive matters were taken in and 

 through which were absorbed and exhaled, oxygen and carbonic acid. 

 Its outer surface was the part which now touched quiescent masses, and 

 now received the collisions consequent on its own motions or the mo- 

 tions of others similarly carried along by their cilia. Its outer surface 

 was the part to receive the sound-vibrations occasionally propagated 

 through the water ; the part to be affected more strongly than any 

 other by those variations in the amounts of light caused by the pass- 

 ing of small bodies close to it ; and the part which met those diffused 

 molecules constituting odours. That is to say, at the outset the sur- 

 face was the part on which there fell the various influences pervading 

 the environment, through which there passed the materials for growth 

 furnished by the environment, by which there were received those im- 

 pressions from the environment serving for the guidance of actions, 

 and which had to bear the mechanical re-actions consequent upon such 

 actions. Necessarily, therefore, the surface was the part in which 

 were initiated the various instrumentalities for carrying on intercourse 

 with the environment. To suppose otherwise is to suppose that such 

 instrumentalities arose internally where they could neither be operated 

 on by surrounding agencies nor operate on them, where the differen- 

 tiating forces did not come into play, and the differentiated structures 

 had nothing to do ; and it is to suppose that meanwhile the parts 

 directly exposed to the differentiating forces remained unchanged. 

 Clearly, then, organization could not but begin on the surface ; and 

 having thus begun, its subsequent course could not but be determined 

 by its superficial origin. And hence these remarkable facts showing 

 us that individual evolution is accomplished by successive in-foldings 

 and in-growings. Doubtless natural selection soon came into action, 

 as, for example, in the removal of the rudimentary nervous centres 

 from the surface ; since an 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 contents initiated. 



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

 factor. Had it done no more than give to Protozoa and Protophyta 

 that cell-form which characterizes them had it done no more than 

 entail the cellular composition which is so remarkable a trait of Meta- 

 zoa and Metaphyta had it done no more than cause the repetition in 

 all visible animals and plants of that primary differentiation 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 organ- 

 isms 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 



