THE FACTORS OP ORGANIC EVOLUTION. 69 



Even neglecting this first stage and commencing with the 

 next, in which a &quot; gastrula &quot; has been produced by the per 

 manent introversion of one portion of the surface of the 

 hollow sphere, it will suffice if we consider what must there 

 after have happened. That which continued to be the outer 

 surface was the part which from time to time touched 

 quiescent masses and occasionally received the collisions 

 consequent on its own motions or the motions of other 

 things. It 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 passing of small 

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

 molecules constituting odours. That is to say, from the 

 beginning the surface was the part on which there fell the 

 various influences pervading the environment, the part by 

 which there was received those impressions from the en 

 vironment serving for the guidance of actions, and the part 

 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 instrumentali 

 ties 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 

 differentiating forces did not come into play, and the differ 

 entiated structures had nothing to do ; and it is to suppose 

 that meanwhile the parts directly exposed to the differentia 

 ting 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 show 

 ing us that individual evolution is accomplished by succes 

 sive 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 



