MARSH WARBLER 



necessary adaptation as the nest-building instinct was suffi- 

 cient to bring about by slow degrees the change which we 

 observe in the nervous system. How can we reconcile these 

 two sets of facts unless we assume that the change of environ- 

 ment did not necessarily precede, nor was coincident with, 

 the transition from one type of nervous system to the other ? 

 Such an assumption may very likely be correct, but we cannot 

 then speak of the environment as either the directing cause or 

 the limiting cause of the specific difference. There is, how- 

 ever, no reason, as has been frequently pointed out, why a 

 variation should not be neutral, should persist, that is to 

 say, and develop so long as it is not harmful. Herein lies a 

 possible explanation of our difficulty. 



Those who are inclined towards the mutation hypothesis 

 will" no doubt ask why one should trouble about these diffi- 

 culties. Natura facit saltum they would say, and what better 

 example could be found of this principle ? It is an attractive 

 solution of our dilemma. We no longer need concern ourselves 

 about the gulf which appears to separate the emotional system 

 of just these or those two closely related forms, and we can 

 even discuss with equanimity the possibility of the change 

 of environment preceding and even acting as a stimulus to 

 the change in the nervous system. The doctrine of mutation 

 assumes that specific differentiation does not take place in any 

 definite direction, that mutations are produced independently 

 of their adaptive value, and that they may survive provided 

 that they do not prejudice the existence or annul the fertility 

 of the individual. The comparisons we have made in the 

 foregoing pages can scarcely be said to afford any direct 

 support for this theory ; they show, it is true, that the 

 relationship between these two species is in some ways difficult 

 to understand if recourse be had solely to continuous variation 

 as an explanation, but to this extent, and this extent only, 

 can they be said to support the mutation hypothesis. 



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