Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/49

 the necessary food, affording protection from cold and from enemies, and attending to the sanitation of the nest as efficiently and rapidly as possible. If then the fæces, instead of being enclosed in this envelope, were ejected in smaller pieces, as is occasionally done even now, what would be the result? Some part of the parents' duty must perforce be neglected; either the young must receive insufficient nourishment, or the nest must become contaminated, or the nestlings must suffer from exposure. The lessening as far as possible of the risk which thus threatens the fragile offspring is, I believe, the biological end for which this envelope has been evolved. But no matter how perfect the system which has been organically built up, the species will not benefit unless the parents, either by the aid of their intelligence, or through acquired experience, or by instinct, are able to take advantage of the special facility for removal which is thus afforded them. I believe that their ability to do so is due to racial preparation. The experiments with small leaves placed in the nest, referred to in the life of the Whitethroat, seem to point to an impulse of considerable strength to remove anything of a foreign nature. For if in place of instinct we fall back upon intelligence, ought we to find leaves carefully picked up and carried away and even efforts made, not without some success, to swallow them? Should we not rather anticipate some small measure of discrimination? The leaf experiments however do not constitute conclusive evidence in favour of something congenital as opposed to something acquired through experience; let us therefore consider the position of a young bird carrying out its parental functions for the first time. How is it to gain its experience? Only, I suppose, by trial and error. But during the process of learning by trial and error what is to happen to the offspring? They must be the sufferers; it is upon them that the initial blunders of an inexperienced parent will recoil, and it is upon them that the future of the race must depend.

How can we account for the parallel development of the