lis ORGANIC EVOLUTION — THE FACTORS 



receive much the same kind and quantity of stimula- 

 tion, direct and indirect, from the environment, and are 

 possessed of much the same power of varying in 

 response to stimulation, as the structures of the other 

 individuals of the species ; as a result corresponding 

 structures in different individuals in corresponding 

 stages of the ontogeny closely resemble one another not 

 only in general form, but also in size. Now since the 

 structures develop mainly in response to stimulation, 

 their development in the later stages of the ontogeny, 

 at any rate, can only be regarded as acquired, not 

 inborn : whence, if it be true that acquired variations 

 are not transmissible, it follows that the later develop- 

 ment cannot be transmitted ; which is in accordance 

 with the facts observed, for in each individual that 

 later development must be acquired anew in response 

 to stimulation, and cannot be achieved without it. 

 That which is transmitted is the power to vary in 

 response to stimulation, and it is inconceivable that this 

 jiower can have resulted from aught but the accumula- 

 tion of inborn variations. 



It follows, moreover, that the adult individuals of 

 any higher species differ among themselves as regards 

 their structures, mainly through differences in this 

 inborn power to vary in response to stimulation. 

 Their structures are similar, but they do not develop 

 to the same extent, or in the same proportion. Indi- 

 viduals differ in size mainly because there is inborn in 

 some or all the structures of the bigger individuals, a 

 greater power to vary in response to stimulation, than 

 there is inborn in the structures of the smaller indi- 

 viduals ; or, to a much less extent, they d iffer in size 

 because the structures of the bigger individuals have 

 received more stimulation than the structures of the 

 smaller individuals : they differ in shape mainly because 

 of differences in the power the structures had to vary 



