MARSH WARBLER
necessary adaptation as the nest-building instinct was sufficient 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 environment 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, however, 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 difficulties. 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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