16 CLASSIFICATION AND ADAPTATION 



considerable range of variation in the specific 

 characters, that, as a rule, no two individuals are 

 exactly alike, even when produced by the same two 

 parents. The central principle of his theory was 

 the survival of individuals possessing those variations 

 which were most useful in the competition of species 

 with species and of individual with individual. He 

 thus explained adaptation to new conditions and 

 divergence of several species from a common 

 ancestor. Characters which were not obviously 

 adaptive were explained either by correlation or 

 by the supposition that they had a utility of which 

 we were ignorant. Darwin also admitted the direct 

 action of conditions as a subordinate factor. 



Weismannism not only retained the principle 

 of utility and selection, but made it the only 

 principle, rejecting entirely the action of external 

 conditions as a cause of congenital modifications, 

 i.e. of characters whose development is pre- 

 determined in the fertilised ovum. It is to 

 Weismann that we owe precise and definite con- 

 ceptions, if not of the nature of heredity, at least of 

 the details of the process. From him we learned to 

 think of the ova or sperms, of the reproductive cells 

 or ' gametes ' of an individual, as cells which were 

 from an early stage of development distinguished 

 from the cells forming the organs and tissues ; 

 to regard the organism as consisting of soma on the 

 one hand and gametes on the other, both derived 

 from the original zygote cell, not the gametes from 

 the soma. Weismann saw no possibility of changes 

 induced by any sort of stimulation in the soma 

 affecting the gametes in such a way as to be re- 

 developed in the soma of the next generation. He 



