A theory of population, deduced from the general law of animal fertility / [Herbert Spencer?].

  • Spencer, Herbert, 1820-1903.
Date:
1852
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Licence: Public Domain Mark

Credit: A theory of population, deduced from the general law of animal fertility / [Herbert Spencer?]. Source: Wellcome Collection.

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    at the nervous systems, however, we find no such discrepancy. On learning' that the average ratio of the brain to the body is— in fishes, 1 to 5668; in reptiles, 1 to 1321; in birds, 1 to 212; and in mammals, 1 to 186 *; their different degrees of fecundity are accounted for. Though an ox will outweigh half-a-dozen men, yet its brain and spinal cord are far less than those of one man; and though in bodily development the elephant so immensely exceeds the human being, yet the elephant’s cerebro-spinal system is only thrice the size attained by that of civilized men f. 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 attainable, they are vitiated by the influence of domes¬ ticity; and partly from the fact that no ]^recise measurement of the respective nervous systems has been made, we are unable to draw any but general and somewhat vague compari¬ sons. 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 spontaneous fission disappears as the nervous system becomes developed; passing again to the least nervous and most fertile of the vertebrate 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 Mam¬ malia, 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 relationship between these attributes. And indeed, on turning back to our a priori principle, no * Quain’s Elements of Anatomy, p. 672. t The maximum weight of the horse’s brain is 1 lb. 7 oz.; the human brain weighs 3 lbs., and occasionally as much as 4 lbs.; the brain of a whale, 75 feet long, weighed 5 lbs. 5 oz.; and the elephant’s brain reaches from 8 lbs. to 10 lbs. 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 less prolific than man. See Muller’s Physiology (Baly’s translation), p. 815, and Quain’s Moments of Anatomyy p. 671.