THEORY OF DEVELOPMENT 397 



of the germ-cells enables us to form a general theory of heredity, 

 enables us to understand how the germ-cells have their peculiar 

 reproducing power. 



A consideration of the facts of inheritance, both general and 

 special, enables us to form a general theory of inheritance — i.e. a 

 speculative thought-model of what the architecture of the 

 germinal material may or must be. 



But it is also necessary to try to form some picture of what 

 occurs during development. The inheritance is in some way 

 expressed, the potentialities are realised, the legacy is cashed — 

 can we form any image of whpt occurs ? As before, our image 

 may not be actually what occurs, but it must not contradict 

 anything that occurs, and, more positively, it must help us to 

 formulate what occurs. This is the business of the theory of 

 development. 



Other Theories are involYed. — The result of development is 

 always an organism more or less like the parent, but the com- 

 pleteness of hereditary resemblance is usually affected by the 

 occurrence of variations, sometimes minute and quantitative, 

 sometimes large and qualitative. It is evident, therefore, that 

 theories of heredity, inheritance, and development must be 

 supplemented by a theory of variation. 



Nor is it possible to abstract the theory of heredity and in- 

 heritance from the theory of growth, reproduction, and sex; 

 from the theory of environmental and functional influences which 

 we sum up in the term " nurture " ; from the theory of the corre- 

 lation of psychical and corporeal life ; and from the general theory 

 of organic evolution in which all biological theories are combined. 



But while we recognise that abstraction of particular problems 

 is merely a device to facilitate clear thinking, and by no means 

 without the counterbalancing dangers which all abstraction 

 involves, we propose in this chapter to restrict our attention 

 to the theories of heredity and inheritance, and to give a 

 general historical retrospect. 



