2 62 THE EVOLUTION OF SEX. 



II. History of Discussion on Rate of Reproduction. — In 



this, as in nut a few other cases, the biologist is profoundly indebted 

 to the student of social questions, for no adequate attenti6n was paid 

 to the laws of multiplication before the appearance of the epoch- 

 making "theory of population" of Malthus, nor is it yet possible or 

 profitable to isolate the human question from the general one. 

 Malthus' s fundamental proposition is indeed usually softened from 

 its earliest form — the population tends to increase in geometrical, 

 subsistence only in arithmetical ratio — into the simple statement that 

 population tends to outrun subsistence, but has none the less served ' 

 as a base of weighty deductions for both the naturalist and the 

 economist. From Darwin's standpoint, the "positive checks" to 

 population (disease, starvation, war, infanticide), and the "pruden- 

 tial ' ' (moral or birth-restricting) checks, come to be viewed as 

 special forms of natural or artificial selection, while the fundamental 

 induction has been extended throughout Nature as the essential 

 condition of the struggle for existence. After long dispute, the 

 induction of Malthus gained acceptance, followed by wide deductive 

 use and abuse, among economists. Yet, fundamentally important as 

 the subject thus is to naturalist and economist alike, the former has 

 not as yet effected any thorough investigation of the conditions of 

 multiplication, or even usually incorporated the keen analysis which 

 we owe to Spencer, while the economic theorist or disputant frequently 

 still employs the doctrine even in its pre-Darwinian form. It is thus 

 doubly needful to summarize, as briefly as maybe, Spencer's elaborate 

 statement of the laws of multiplication. 



III. Summary of Spencer's Analysis. — Different species exhibit different 

 degrees of fertility, which have become established in process of evolution like 

 the organisms themselves. To understand this particular adaptation of function 

 to conditions of existence, of organism to environment, we may analyze these 

 into their respective factors. It is evident that in the environment of any species 

 there are many conditions with which its individuals 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 overthrown, — 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 environment and organism admit of another grouping in more 

 familiar terms, into two conflicting sets, — (a) the forces destructive of race ; (b) 

 the forces preservative 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 



