No. 579] PROGRESSIVE EVOLUTION 167 



the phylogenetic point of view, can be nothing else but 

 the sum of its successive terms. 



It will be convenient, before going further, to sum up 

 the results at which we have so far arrived from the 

 point of view of the theory of heredity. We have as yet 

 seen no reason to distinguish between somatogenic and 

 blast ogenic characters. All the characters of the adult 

 animal are acquired during ontogeny as the result of 

 the reaction of the organism to environmental stimuli, 

 both internal and external. All that the organism ac- 

 tually inherits is a certain amount of protoplasm — en- 

 dowed with a certain amount of energy — and a certain 

 sequence of environmental conditions. In so far as 

 these are identical in any two successive generations the 

 final result must be identical also, the child must re- 

 semble the parent; in so far as they are different the 

 child will differ from the parent, but the differences in 

 environment can not be very great without preventing 

 development altogether. 



So far, it is clear, there has been no need to think of 

 the germ-cells as the bearers of material factors or de- 

 terminants that are responsible for the appearance of 

 particular characters in the adult organism; nor yet to 

 suppose that they are, to use the phraseology of the 

 mnemic theory of heredity, charged with the memories 

 of past generations. They have been regarded as simple 

 protoplasmic units, and the entire ontogeny has appeared 

 as the necessary result of the reaction between the or- 

 ganism and its environment at each successive stage of 

 development. This can not, however, be a complete ex- 

 planation of ontogeny, for if it were we should expect all 

 eggs, when allowed to develop under the same conditions 

 from start to finish, to give rise to the same adult form, 

 and this we know is not the case. We know also, from 

 observation and experiment, that the egg is in reality by 

 no means a simple thing but an extremely complex one, 

 and that different parts of the egg may be definitely cor- 

 related with corresponding parts of the adult body. It 



