The Theory of Evolution 229 



Many will be incapable of surviving, however often they appear, 

 and even some kinds that might survive may be swallowed up 

 again, so to speak, by the parent species. 



Many further questions concerning the factors of evolution 

 suggest themselves which cannot be answered at present. For 

 example : if a mutation arises and survives, is another mutation 

 in the same direction more likely than in a different direction? 

 If a mutation is traceable directly to external factors acting on an 

 internal condition or at a particular stage of the germ-cells, will 

 it recur again and again in all individuals, subjected to the same 

 conditions, or will it be confined to the descendants of those strains 

 in which they have first "accidentally" (?) arisen? What pro- 

 portion of mutations survive autonomously and what proportion 

 by crossing with the parent stock ? How many are lost by being 

 recessive, and how often do these recessive individuals form ele- 

 mentary varieties ? Do progressive changes take place that are 

 definite (i.e. not fluctuating) in character and are too small 

 for us to recognize them as mutations because the steps fall 

 within the range of fluctuating variations ? 



These and many other questions demand to be answered 

 by any one who attempts to apply the observed facts of muta- 

 tion and discontinuous inheritance to the theory of evolution. 

 It is obvious that until we can answer them we must remain 

 in the dark concerning the influence on evolution, and can 

 only suggest, but not prove, that mutations have furnished 

 some of the materials for organic evolution. 



Adaptation 



The problem of evolution of organisms has become so closely 

 associated with the question of adaptation that we must briefly 

 refer to this question in connection with the mutation theory. 

 If a species could be changed (so that it became a new species 

 adapted to a new environment) by picking out those fluctuating 

 variations of an adaptive kind, the problem of adaptation would 

 occupy an important place in experimental zoology. But if 

 this is not the case, the question of adaptation occupies a secon- 



