If, then, every male is driven by inherited impulse to seek the appropriate breeding ground each recurring season; if, having arrived there, it is driven to seek a position of its own; if, in order to secure isolation it is obliged to attack other males or to ward off the attacks of intruders; if, in short, success can only be attained providing that the inherited nature is so adjusted that the bird can accomplish all that is here demanded--what will be the general result? That the individual will rear its offspring in safety and that they will inherit the peculiarities of their parents, enabling them, in their turn, to procreate their kind; all this will certainly follow. We are not concerned, however, at the moment, with the direct effect upon the individual, but with the consequences that will accrue to the species as a whole. Now certain facts are presented to observation which enable us not only to understand the nature of the change that is wrought in the history of the species, but to foreshadow, with no small degree of certainty, the extent of that change. I suppose that it has come within the experience of most of us to observe, at one time or another, the ebb and flow of a given species in a given district. Some favourite haunt is deserted for a year, or for a term of years, and is then revisited; or, if it is always occupied, the number of inhabitants fluctuates--plenty of pairs in this season, only a few in that. Many intricate relationships, both external and internal, contribute towards this state of affairs. Fluctuation in a downward direction, or temporary extinction, is brought about by changes in the physical world, by changes in the available supply of food, by the increase of enemies, or by adverse climatic conditions; whilst fluctuation in an upward direction, though due indirectly to a combination of circumstances in the external world favourable to the survival of large numbers of individuals, is directly determined by the impulse to seek isolation. As individuals of different species establish themselves, and form kingdoms and lesser kingdoms, we can watch the gradual quickening into life of moorland and forest and we can observe the manner in which it all comes to pass. Males that for weeks or months have lived in society, drifting from locality to locality according to the abundance of food or its scarcity, now set forth alone and settle first here and then there in search of isolation. Lapwings settle in the water meadows, and, finding themselves forestalled, pass on in search of other ground; Blackbirds arrive in a coppice or in a hedgerow and, meeting with opposition, disappear; and the Curlew, wandering with no fixed abode but apparently with a fixity of purpose, searches out the moorland where it can find the particular environmental conditions to which its inherited nature will respond. In fact, wherever we choose to look, we can observe in a general way the