For "self-preservation" and "self-development," though frequently spoken of as identical, are really distinct and often opposed notions--the former denoting a tendency to persist in one's present state of being, while the latter implies more or less change. It may be held, however, that for an organism such as man to persist in his state of being, implies modification of his faculties, and that this modification involves development. For any organism to exist apart from change is, of course, impossible. Life is only known to us as a series of changes. But that change does not necessarily mean development or "change to a higher condition." Degradation is as well known a fact as development; and between the two, there is room for a state of existence of which it is difficult to say whether it improves or deteriorates. And whatever may be intended by the phrase, "self-preservation" points to a state of this kind rather than to an improving condition. The notion of "self-development" has therefore a richer content than that of "self-preservation"; but just on this account it cannot be explained by a reference to the nature of things as they are. [Sidenote: thus taking account of variability] It is true that self-development can only go on by a continuous process of adjustment; but it is also necessary for it that this tendency to adaptation should be continually hindered from becoming complete or lapsing into equilibrium. It is here that the function of variation comes in. On the one side there is this tendency to vary after a fashion often without any apparent regard to external conditions; on the other side, there is the action of the external conditions selecting and favouring those variations which bring the organism into closer correspondence with them. The wide range over which the theory of natural selection applies is due to the fact that the environment is never uniform and never constant, so that modifications on the part of the organism have a chance of suiting its varied and changing character. Its changes, moreover, are often the result not so much of any absolute alteration in external circumstances, as of a new relation between them and living beings having been brought about. For the enormous reproductive faculty of most organisms makes them multiply so rapidly as to press ever more and more closely against the limit of subsistence, and thus to produce competition for the means of living. Hence the fresh lines of development originated by each organism have to be tested by their correspondence with a constantly changing medium. The altered circumstances give the modifications which organisms are for ever striking out an opportunity of perpetuating themselves. [Sidenote: which complicates the tendency to correspondence with environment,]