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

  • Spencer, Herbert, 1820-1903.
Date:
1852
Catalogue details

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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    Still more clearly is this truth illustrated throughout the animal kingdom. Though not so great as the fertility of the Protophyta, which, as Prof. Henslow says, in some cases passes comprehension, the fertility of the Protozoa is yet almost beyond belief. In the polygastric animalcules spontaneous fission takes place so rapidly that it has been calculated by Prof. Ehren- berg that no fewer than 268 millions might be produced in a month from a single Paramecium and even this astonishing rate of increase is far exceeded in another species, one individual of which, only to be perceived by means of a high magnify¬ ing power, is calculated to generate 170 billions in four days.’^T Amongst the larger organisms exhibiting this lowest mode of reproduction under a modified form—that of gemmation—we see that, though not nearly so rapid as in the Infusoria, the rate of multiplication is still extremely high. This fact is well illus¬ trated by the polypes; and in the apparent suddenness with which whole districts are blighted by the Aphis (multiplying by internal gemmation), we have a familiar instance of the start¬ ling results which the parthenogenetic process can achieve. Where reproduction becomes occasional instead of continuous, as it does amongst higher creatures, the fertility equally bears an inverse ratio to the development. The queen ant of the African Termites lays 80,000 eggs in twenty-four hours; and the common hair worm {Gordius) as many as 8,000,000 in less than one day.^'J Amongst the Vertehrata the lowest are still the most prolific. It has been calculated,^’ says Carpenter, that above a million of eggs are produced at once by a single cod¬ fish.” § In the strong and sagacious shark comparatively few are found. Still less fertile are the higher reptiles. And amongst the Mammalia, beginning with small Rodents, which quicfly reach maturity, produce large litters, and several litters in the year; advancing step by step to the higher mammals, some of which are long in attaining the reproductive age, others ot which produce but one litter in a year, others but one young one at a time, others who unite these peculiarities; and ending with the elephant and man, the least prolific of all, we find that throughout this class, as throughout the rest, ability to mul¬ tiply decreases as ability to maintain individual life increases. § 4. The a priori principle thus exemplified has an obverse of a like axiomatic character. We have seen that for the continu¬ ance of any race of organisms it is needful that the power of * Prin. of Phys., 3rd edit. p. 249. t Ibid., p. 124. $ Agassiz and Gould, p. 274. § Prin. of Pbys., 3rd edit. p. 964.