LAWS OF MULTIPLICATION. 2 85 



establish a moving equilibrium, sooner or later overthrown in death. To 

 prevent extinction, the organism meets these environing actions in two 

 distinct ways, — (i) by individual adaptations, active thrusts or passive 

 parries ; (2) by the production of new individuals to replace those over- 

 thrown, — in other words, by genesis. The latter may occur, as we have 

 seen, in varied forms, sexual or asexual, and at various rates, which depend 

 upon age, frequency, fertility, and duration of reproduction, together with 

 amount and nature of parental aid. These actions and reactions of environ- 

 ment and organism admit of another grouping in more familiar terms, into 

 two conflicting sets, — {a) the forces destructive of race ; {b) the forces pre- 

 servative of race. 



Leaving aside cases in which permanent predominance of destructive 

 forces causes extinction, and also, as infinitely improbable, cases of perfectly 

 stationary numbers, the inquiry is : — In races that continue to exist, what 

 laws of numerical variation result from these variable conflicting forces 

 that are respectively destructive or preservative of race? How is the 

 alternate excess of one or other rectified ? A self-sustaining balance must 

 exist ; the alternate predominance of each force must initiate a compensa- 

 tory excess of the other ; how is this to be explained ? 



When favourable circumstances cause any species to become unusually 

 numerous, an immediate increase of destructive influences, passive as well 

 as active, takes place ; competition becomes keener and enemies more 

 abundant, and conversely. Yet this is not the sole, much less the perma- 

 nent, means of establishing a balance ; nor does it explain either the 

 differences in the rate of fertility and mortality, or the adaptation of one to 

 the other. This minor adjustment in fact implies a major one. 



The forces preservative of race were seen above to be two, — power to 

 maintain individual life, and power to generate the species. Now, in a 

 species which survives, given the forces destructive of race as a constant 

 quantity, those preservative of race must be a constant quantity also ; and, 

 since the latter are two, the individual plus the reproductive, these must 

 vary inversely, one must decrease as the other increases. To this law 

 every species must conform, or cease to exist. Let us restate this at greater 

 length. A species in which self-preservative life is low, and in which the 

 individuals are accordingly rapidly overthrown in the struggle with the 

 destructive forces, must become extinct, unless the other race-preservative 

 factor be proportionally strengthened, — unless, that is to say, its reproductive 

 power become proportionally great. On the other hand, if both preserva- 

 tive factors be increased, if a species of high self-preservative power were 

 also endowed with powers of multiplication beyond what is needful, such 

 success of fertility, if extreme, would cause sudden extinction of the species 

 by starvation, and if less extreme, and so effecting a permanent increase of 

 the numbers of the species, would next bring about such intenser competi- 

 tion, such increased dangers to individual life, that the great self-preserva- 

 tive power would not be more than sufficient to cope with them. 



In short, then, we have reached the a priori principle, that in races 

 which continuously survive, in which the destructive forces are balanced 

 by the preservative ones, there must be an inverse proportion between the 

 power to sustain individual life and the power to produce new individuals. 

 But what is the physiological explanation of this adjustment, and how has 

 it arisen in process of evolution ? Spencer has elsewhere enlarged upon the 

 proposition, which we have already illustrated, that genesis in all its forms 



