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 zoédlogy. But if 
this is not the case, the question of adaptation occupies a secon- 
