Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/55

 to some external influence, or to internal variations of the nervous system of definite or indefinite origin, we cannot say.

On more than one occasion, I have appealed to natural selection as a probable explanation of the method by which some particular piece of adaptation has been built up. One of the criticisms to which this theory is sometimes subjected is that we ought to be able to observe it in operation providing it were of such universal application. Much depends upon how much we expect to see. We cannot of course hope, within the short space of a human lifetime, to witness the completion of a process, to see some great change accomplished by virtue of some specific inherited disposition; but because we cannot be present while the whole wall is being built we are not thereby debarred from seeing some of the bricks laid. Wherever we find failure—failure to escape the dangers attendant on migration, failure to secure food under normal conditions, failure to attain the goal of reproduction—there we have elimination with the corresponding brick laid in the development of some characteristic which is of survival value. Take as an example the nesting instinct of the Reed Warbler. These birds weave their nests to three, four, five, six or seven reeds; two would clearly be insufficient to hold the nest; three do not always afford the necessary security, the majority are therefore woven to more. We can observe that a nest woven to three reeds is liable to bring disaster to the offspring, though perhaps not often, and we can infer that if it were woven to two that danger would be increased. Now it is more simple to find two reeds conveniently placed than to find three, more simple to find three than four. If then the number of reeds made use of were a matter of small moment, why should the number of nests woven to more than three be in so large a preponderance? What determines individual behaviour in this respect? Does the same individual make use of two reeds one year and fail, but five reeds the next and succeed? Or do some individuals always lay the foundations of their nest more securely than others? Since experience as