been an ever-widening one; and as with their multiplication, irregularities and delays in mating, arising from the similarity of the calls, would have increased in frequency, so a distinctive call, which would have tended to minimise these risks, would have come to possess biological value.
Here we have a theory of origin, but origin of what? Of certain characteristics of song—nothing more; and therefore to suppose that it furnishes a complete explanation, which satisfies all the requirements of scientific logic, of so wonderful an intonation as that, for example, of the Marsh-Warbler, or that no other relationships, except that of the territory, enter into the total emotional complex, simplifying here or elaborating there to meet the exigencies of diverse circumstances—to suppose this would be foolish. That there are many relationships which even to-day are leading to modifications in important particulars, but which at the present time are beyond our cognisance, of this there can be no doubt.
There is one process by which song may have attained a fuller development, and which would account in some measure for the elaboration, inexplicable merely in terms of "recognition." It is this: the effect of the sexual call upon the female cannot well be neutral, it must be either pleasurable or the reverse—it must, that is to say, be accompanied by some suggestiveness, and by suggestion I, mean the arousing of some emotion akin to that of the male; and if there
