246 Darwin, and afiei Darwin. 



of obtaining " evidence " ,pon the subject. But we 

 are not on this account entitled to conclude against 

 the probability of such changes of specific type 

 having been more or less frequently thus produced. 

 And still less can we be on this account entitled to 

 conclude against the possibility of such a change 

 having ever occurred in any single instance. Yet 

 this is what must be concluded by any one who 

 maintains that the origin of all species and, a for- 

 tiori, of all specific characters must necessarily have 

 been due to natural selection. 



Now, if all this be admitted and I do not see how 

 it can be reasonably questioned consider how impor- 

 tant its bearing becomes on the issue before us. If 

 germ-plasm (using this term for whatever it is that 

 constitutes the material basis of heredity) is ever 

 capable of having its congenital endowments altered 

 by the direct action of external conditions, the result- 

 ing change of hereditary characters, whatever else 

 it may be, need not be an adaptive change. Indeed, 

 according to Weismann's theory of germ-plasm, the 

 chances must be infinitely against the change being 

 an adaptive one. On the theory of pangenesis that 

 is to say, on the so-called Lamarckian principles 

 there would be much more reason for eiicertaining the 

 possibly adaptive character of hereditary change due 

 to the direct action of the environment. Therefore 

 we arrive at this curious result. The more that we are 

 disposed to accept Weismann's theory of heredity, and 

 with it the corollary that natural selection is the sole 

 cause of adaptive modification in species, the less are 

 we entitled to assume that all specific characters 

 must necessarily he adaptive. Seeing that in nature 



