480 LAWS OF MULTIFJ ICATION. 



activity, continuing during the prime of life, ceases when the 

 vigour declines, leaving a closing period of infertility; and in 

 like manner among ourselves, barrenness supervenes when 

 middle age brings the surplus vitality to an end. So, too, 

 it is found that in Man, as in beings of lower orders, there is 

 a period at which fecundity culminates. In § 341, facts were 

 cited showing that at the commencement of the reproductive 

 period, animals bear fewer offspring than afterwards ; and 

 that towards the close of the reproductive period, there is a 

 decrease in the number produced. In like manner it is shown 

 by the tables of Dr. Duncan's recent work, that the fecundity 

 of women increases up to the age of about 25 years ; and 

 continuinof hio:h with but slight diminution till after 30, 

 then gradually wanes. It is the same with the sizes and 

 M^eights of offspring. Infants boin of women from 25 to 29 

 years of age, are both longer and heavier than infants born 

 of vounger or older women ; and this difference has the same 

 implication as the greater total weight of the offspring pi'o- 

 duced at a birth, during the most fecund age of a pluriparous 

 animal. Once more, there is the fact that a too-early bearing 

 of young produces on a woman the same injurious effects as 

 on an inferior creature — an arrest of growth and an enfeeble- 

 ment of constitution. 



Considering these general and special parallelisms, we 

 might safely infer that variations of human fertility conform 

 to the same laws as do variations of fertility in general. 

 But it is not needful to content ourselves with an implication. 

 Evidence is assignable that what causes increase or decrease 

 of orenesis in other creatures, causes increase or decrease of 

 genesis in Man. It is true that, even more than hitherto, our 

 reasonings are beset by difficulties. So numerous are the 

 inequalities in the conditions, that but few unobjectionable 

 comparisons can be made. The liuman races differ consider- 

 ably in their sizes, and notably in their degrees of cerebral 

 development. The climates they inhabit entail on them 

 widely different consumptions of matter for maintenance of 



