Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/55

. Thus the range would be gradually extended further and further from its base until it began to advance into regions subject to alternations in climate, and in which, consequently, the duration of food supply was more or less limited to a few months in the year. Whenever this point was reached, it would be necessary for the individuals to return for part of the year within the zone of perpetual food supply. Now we know that, in the case of the migratory species, there is a strong propensity in the individual to revisit in the following breeding season that particular neighbourhood in which it was reared. But even if we did not know this we should infer that it was so from our knowledge of the conditions under which the range of the species is being extended, i.e., from the law of territory. For if those males that were forced to desert a certain district through insufficient food supply during part of the year did not again go forward the following spring to their former breeding haunts, overcrowding would occur at some point; and inasmuch as reproduction would be impossible to a large and increasing number, owing to lack of territory, the distribution would remain stationary, and a check would be administered to the welfare of the species. Therefore any variation in the direction of a tendency to return would be fostered and developed by selection. And so it would be with the females; those that did not return to their birthplace, or their old breeding haunts, would be less likely to reproduce, since they would have greater difficulty in finding an unpaired male in possession of a territory. We can then understand that the range of our imaginary species would be extended with a corresponding increase in the distance traversed by some of its individuals between the zone of perpetual food supply and the breeding haunts, until the limit imposed by unfavourable conditions of existence were reached. And when we consider the conditions under which it must have existed, the constant struggle and constant search for new breeding grounds, changes in the earth's surface, and changes of climate, resulting in the gradual disappearance of