BRITISH WARBLERS
what exactly do we mean? That the initial efforts of the young bird to rear its offspring are doomed to failure? Hardly, one would think, if the preservation of the race is to be secured. The nest has a very definite biological end to serve; it matters not whether it is a little more or a little less highly elaborated; what matters is that it should be sufficiently stable to fulfil the requirements of incubation and ensure the safety of the offspring. Those individuals that failed in the first attempt, even though their failure was tempered by success on subsequent occasions, could have no chance with others in whom the instinct was perfect from birth, and must have long since been eliminated. Moreover no increase of skill begotten of experience, which would lead to a greater perfection of detail, a more highly elaborated production over and above that which was necessary for success, could benefit the offspring. One finds nests, it is true, which seem to bear traces of imperfect workmanship, but they have generally followed the destruction of a first attempt and are built therefore hurriedly to receive the eggs already overdue; yet even they, unfinished though they may be in workmanship, meet the biological end of nest building. I do not deny the existence of nests ill adapted to their surroundings. A change of environment is all that is required to supply the conditions necessary for the appearance of individual variations in constructive ability, some of which will disappear, while others more in harmony with their surroundings will remain; and I have given my reasons in the history of the Marsh Warbler for believing that such a change of environment is actually in progress at the present time in the case of that particular species. There, as a reference to the illustrations will show, we have different types of nests which seem to bear the stamp of varying degrees of constructive skill, and all will perhaps in the future become modified before the evolution is complete, since they appear to be either too flimsy or too robust.
Others regard the passing on of experience by the elder to
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