GENERAL LAW OF ANIMAL FERTILITY. 599 



men.* Unfortunately, it is impossible to trace throughout the 

 animal kingdom this inverse relationship between the nervous 

 and reproductive systems with any accuracy. Partly from the 

 fact that, in each case, the degree of fertility depends on three 

 variable elements the age at which reproduction begins, the 

 number produced at a birth, and the frequency of the births; 

 partly from the fact that, in respect to most animals, these data 

 are not satisfactorily attainable, and that, when they are attain- 

 able, they are vitiated by the influence of domesticity ; and parti v 

 from the fact that no precise measurement of the respective 

 nervous systems has been made, we are unable to draw any but 

 general and somewhat vague comparisons. These, however, as 

 far as they go, are in our favour. Ascending from beings of the 

 acrite nerveless type, which are the most prolific of all, through 

 the various invertebrate sub-kingdoms, amongst which spontane- 

 ous fission disappears as the nervous system becomes developed ; 

 passing again to the least nervous and most fertile of the verte- 

 brate series Fishes, of which, too, the comparatively large- 

 brained cartilaginous kinds multiply much less rapidly than the 

 others ; progressing through the more highly endowed and less 

 prolific Reptiles to the Mammalia, amongst which the Rodents, 

 with their unconvoluted brains, are noted for their fecundity ; and 

 ending with man and the elephant, the least fertile and largest- 

 brained of all there seems to be throughout a constant relation- 

 ship between these attributes. 



And indeed, on turning back to our a priori principle, no other 

 relationship appears possible. We found it to be the necessary 

 law of maintenance of races, that the ability to maintain indi- 

 vidual life and the ability to multiply vary inversely. But the 

 ability to maintain individual life is in all cases measured by the 

 development of the nervous system. If it be in good visceral organi- 

 zation that the power of self-preservation is shown, this implies 

 some corresponding nervous apparatus to secure sufficient food. 

 If it be in strength, there must be a provision of nerves and 

 nervous centres answering to the number and size of the muscles. 

 If it be in swiftness and agility, a proportionate development of 

 the cerebellum is presupposed. If it be in intelligence, this varies 



* The maximum weight of the horse's brain is 1 Ib. 7 ozs. ; the human 

 brain weighs 3 Ibs., and occasionally as much as 4 Ibs. ; the brain of a 

 whale, 75 feet long, weighed 5 Ibs. 5 ozs. ; and the elephant's brain reaches 

 from 8 Ibs. to 10 Ibs. Of the whale's fertility we know nothing; but the 

 elephant's quite agrees with the hypothesis. The elephant does not attain its 

 full size until it is thirty years old, from which we may infer that it arrives 

 at a reproductive age later than man does ; its period of gestation is two 

 years, and it produces one at a birth. Evidently, therefore, it is much loss 

 prolific than man. Sec MiillerV I'hi/slology (Baly's translation), p. 815, and 

 Quain's Elements of Anatomy, p. 071. 



