DEVELOPMENT BY NATURAL SELECTION. 121 



is that in the adaptation the identity of the organism 

 is preserved. We can always trace in the unadapted 

 organism the rudiments of what has become more evident 

 in the adapted organism. In this case there is develop- 

 ment, and not mere change in the physical sense. Along- 

 side of what is ordinarily called development, there may 

 occur the converse process of degeneration, where some 

 structure or capacity which was well developed before 

 has become more rudimentary ; and this, of course, is 

 a common occurrence. In man, for instance, the organs 

 and capacities connected with smell have become com- 

 paratively rudimentary ; and in the individual man 

 excessive specialisation in one direction is apt to be at 

 the expense of atrophy of other -capacities. But the 

 maintenance of physiological and structural identity is 

 no less evident in degeneration than in development. 



Variation of all kinds in the individual organism seems 

 to be of the nature of development or degeneration. 

 This implies that in all variation the specific nature of 

 the organism expresses itself. The new environment to 

 which the variation is a response has definitely become, 

 as it were, a part of the organism's normal environment, 

 and so an element in the unity which, as already explained, 

 constitutes its life. 



According to the hypothesis put forward by Weis- 

 mann, and adopted by many biologists, acquired char- 

 acters are not transferred from the parent organism to 

 the germ-cells, and are consequently not inherited. I 

 have already pointed out that this hypothesis does not 

 simplify in any way the problem of heredity. There is 

 also direct experimental evidence against it, which, 

 however, I can hardly discuss at present. What I do 



