this, that appropriate dissection reveals in most of the migrants, upon arrival at their destination, unquestionable evidence of seasonal increase in the size of the sexual organs. Beyond this there is nothing to go upon. Yet if the term "sexual instinct" is held to comprise the whole series of complex relationships which are manifest to us in numerous and specialised modes of behaviour, which ultimately lead to reproduction, and which have gradually become interwoven in the tissue of the race, there can be little doubt that the assumption is a reasonable one. To some, the term may recall the fierce conflicts which are characteristic of the season; to others, emotional response; to not a few, perhaps, the actual discharge of the sexual function—all of these, it is true, are different aspects of the one instinct; but at the same time each one marks a stage in the process, and the different stages follow one another in ordered sequence. However, we are not concerned at the moment with the term in its wider application; we wish to know the precise stage at which the disposition to mate influences the behaviour of the male. Is the female to him, from the moment the seasonal change in his sexual organs takes place, a goal that at all costs must be attained? Or is it only when the cycle of events which leads up to reproduction is nearing completion that she looms upon his horizon? One would like to be in a position to answer these questions, but there is nothing in the way of experimental evidence to go upon; and if I say that there is reason to believe that, in the earlier stages, the female is but a shadow in the external environment of the male, it must be taken merely as an expression of opinion, though based in some measure upon a general observation of the behaviour of various species.

Before attempting to explain the difference in the times of arrival of the male and female migrant, let us examine the behaviour of some resident species at a corresponding period. My investigations have been made principally amongst the smaller species—the Finches and the Buntings—which often pass the winter in or near the localities wherein they brought up offspring or were reared. It is true that they wander from one field to another according to the abundance or scarcity of food; it is also true that, if the weather is of a type which precludes the possibility of finding the necessary food, these wanderings may become extensive or even develop into partial migrations. But under the normal climatic conditions which prevail in many parts of Britain, these smaller resident species seem to find all that they require without travelling any great distance from their breeding haunts. Flocks composed of Yellow Buntings, Cirl Buntings, Corn-Buntings, Chaffinches, Greenfinches, etc., can be observed round the farmsteads or upon arable land; small flocks of Reed-Buntings take up their abode on pieces of waste land and remain there until the supply of food is exhausted, deserting their feeding ground only towards evening when they retire to the nearest reed-bed to pass the night; flocks of Hawfinches visit the same holly-trees day after day so long as there is an abundance of berries on the ground beneath; and so on.

I have mentioned the Reed-Bunting; let us take it as our first example and try to follow its movements when the influence exerted by the internal secretions begins to be reflected on the course of its behaviour. First, it will be necessary to discover the exact localities in any given district to which the species habitually returns for the purpose of procreation; otherwise the earlier symptoms of any disposition to secure a territory may quite possibly be overlooked in the search for its breeding haunts.

In open weather Reed-Buntings pass the winter either singly, in twos or threes, or in small flocks, on bare arable ground, upon seed fields, or in the vicinity of water-courses; but in the breeding season they resort to marshy ground where the Juncus communis grows in abundance, to the dense masses of the common reed (Arundo phragmites), and such like places. During the winter, the male's routine of existence is of a somewhat monotonous order, limited to the necessary search for food during the few short hours of daylight and enforced inactivity during the longer hours of darkness. But towards the middle of February a distinct change manifests itself in the bird's behaviour. Observe what then happens. When they leave the reed-bed in the morning, instead of flying with their companions to the accustomed feeding grounds, the males isolate themselves and scatter in different directions. The purpose of their behaviour is not, however, to find fresh feeding grounds, nor even to search for food as they have been wont to do, but rather to discover stations suitable for the purpose of breeding; and, having done so, each male behaves in a like manner—it selects some willow, alder, or prominent reed, and, perching thereon, leads a quiet life, singing or preening its feathers. Now if the movements of one particular male are kept in view, it will be noticed that only part of its time is spent in its territory. At intervals it disappears. I do not mean that one merely loses sight of it, but that it actually deserts its territory. As if seized with a sudden impulse it rises into the air and flies away, often for a considerable distance and often in the same direction, and is absent for a period which may vary in length from a few minutes to an hour or even more. But these periodical desertions become progressively less and less frequent in occurrence until the whole of its life is spent in the few acres in which it has established itself.

The behaviour of the Yellow Bunting is similar. In any roadside hedge two or more males can generally be found within a short distance of one another, and in such a place their movements can be closely and conveniently followed. Under normal conditions the ordinary winter routine continues until early in February; but the male then deserts the flock, seeks a position of its own, and becomes isolated from its companions. Now the position which it selects does not, as a rule, embrace a very large area—a few acres perhaps at the most. But there is always some one point which is singled out and resorted to with marked frequency—a tree, a bush, a gate-post, a railing, anything in fact which can form a convenient perch, and eventually it becomes a central part of the bird's environment. Here it spends the greater part of its time, here it utters its song persistently, and here it keeps watch upon intruders. The process of establishment is nevertheless a gradual one. The male does not appear in its few acres suddenly and remain there permanently as does the migrant; at first it may not even roost in the prospective territory. The course of procedure is somewhat as follows:—At dawn it arrives and for a while utters its song, preens its feathers, or searches for food; then it vanishes, rising into the air and flying in one fixed direction as far as the eye can follow, until it becomes a speck upon the horizon and is ultimately lost to view. During these excursions it rejoins the small composite flocks which still frequent the fields and farm buildings. For a time the hedgerow is deserted and the bird remains with its companions. But one does not have to wait long for the return; it reappears as suddenly as it vanished, flying straight back to the few acres which constitute its territory, back even to the same gate-post or railing, where it again sings. This simple routine may be repeated quite a number of times during the first two hours or so of daylight, with, of course, a certain amount of variation; on one occasion the bird may be away for a few minutes only, on another for perhaps half an hour, whilst