effective pairing situation, it is serviceable in furthering the life of the individual. I do not propose at the moment to inquire whether this doctrine be true, but rather to direct attention to other ways in which the song may be useful.
Is the instinct susceptible to stimulation under all conditions during the season of reproduction, or only under some well defined condition? This is the question to which we will first direct inquiry.
Song in its full development belongs, as we have seen, to the season of reproduction; it is heard at the dawn of the seasonal sexual process, and is the most conspicuous outward manifestation of the internal organic changes which ultimately lead to reproduction. These changes would appear, at first sight, to be the primary condition which renders the instinct, susceptible to appropriate stimulation. But while this is true up to a point, in so far, that is to say, as organic changes are a necessary antecedent of all behaviour connected with the attainment of reproduction, closer acquaintance with the circumstances under which the instinct is allowed full play leads to the belief that they are not alone sufficient to account for the facts as observed. In order to arrive at a decision we must seek out the specific factors in the external environment with which "song" is definitely related.
Some birds cross whole continents on their way to the breeding grounds, others travel many
