Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/92

 the two, since the possession of certain congenital nervous dispositions determines the actions which are adapted to promote the welfare of the species; but of the nature of that relation we are ignorant. Did the change of environment precede, was it coincident with, or subsequent to the change in the nervous system? Our answer to this question will depend in a great measure upon the view we hold with regard to the development of living forms from others of a simpler type. From our inquiries into the life-histories of the two species we believe that they may have been at some earlier time even more closely related than they are to-day, and we believe that the Marsh Warbler is the more recent modification. According to the theory of continuous variation this result must have been brought about by a number of successive steps or gradations, each one of which possessed some definite value in the struggle for existence and was thus left free to develop. And those who hold that this is the only means by which one form can have passed into another will point to the variation in the nesting instinct in proof of their assertion; and doubtless they will be justified in doing so, for there we have indisputable evidence of the process at work. But the fact of the nesting instinct being in a condition of instability shows that sufficient time has not yet elapsed to complete the adaptation, and consequently the change from one environment to the other must be regarded from this point of view as of more or less recent occurrence. A difficulty arises here. The vocal powers and motor reactions of the two species are separated as regards development by a very considerable interval, and in the case of the Marsh Warbler they are not subject to variation to such an extent as the nesting instinct but seem to have reached a condition of comparative stability. On the hypothesis of gradual transition such an interval must represent an infinite number of gradations, which, in their turn, must have required a long period of time for their completion, and I find it difficult to imagine that time which was insufficient to complete such a simple but nevertheless very