MARSH WARBLER
tion of the difference lies outside the province of utility indicated by Professor Groos; there must be some other factor which, exercising independent control, arrests development here or allows it to proceed there to the verge of extravagance. I therefore suggest that the reactions may be by-products of this factor which regulates the intensity of the activity feelings, and that after all there may be something to be said for the third proposition—the view that would regard them as having arisen independently of any question of utility. The infinite number of specific types of reaction lends some support to this view. How frequently we can distinguish a species merely by some particular way in which it behaves in the breeding season! The Greenfinch, Yellow Bunting[1], Tree Pipit, Meadow Pipit, Snipe, Ruff, Redshank, Black-tailed Godwit and a host of others we can recognise, each one by its own particular mode of conduct. The reactions reflect the emotions—says the " utility " hypothesis—excite the female and are thus of some use, but why should they have taken the form of a spreading of the tail in this case, a waving of wings in that, or a fluttering in mid-air in a third? It is evident that there is something here which requires an explanation if we regard them thus, and it is evident that the assistance of some further factor, which can guide the reactions into certain definite channels, must be assumed in order to complete the explanation. In one passage Professor Groos seems to recognise the difficulty of attributing utility to the reactions and almost approaches the conclusion that they may be but incidents of the nervous system having no special part to fulfil. [2]" We must admit," he says, "that in most cases the actual basis for the arts of courtship is to be found in general excitement reflexes, or even in those of quite a different origin. This basis consists partly in such reflex motions as result from any strong excitation such as restless fluttering, running about, skipping and
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