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it is not at all to be wondered at that such individuals become sterile. Most birds are rendered incapable of procreation by captivity; and a large number of animals are never known to breed when confined in dens or even in paddocks. Hence such facts cannot be admitted as evidence of a doctrine depending mainly on a false analogy between plants, which are capable of reproduction in two ways, and animals, which procreate in one manner only. No one thinks of denying that the size of a tree depends on the amount of nourishment afforded to it by surrounding circumstances; and this is a tacit admission that reproduction by buds takes place in a ratio directly as the amount of material assimilated. Nor is it believed by any gardener or naturalist that a starved flower produces more seeds than a well-nourished one ; indeed the number of well-formed seeds, other things being equal, is in a direct ratio to the degree of nutrition afforded to the seed-capsule or ovary.
The increase of all living beings proceeds in a geometrical ratio ; and this follows as a direct result of the fact that each parent, or pair of parents, produces several young, each of which, or each pair of which, become mature and multiply in the same manner. This method of increase is checked by the fact that only a limited number of individuals can live in a given area ; so that in an
INCREASE DIRECTLY AS THE FOOD SUPPLIED. 87
already stocked country only two offspring can survive for each pair of parents. The struggle for existence between the offspring is the result of their early separation from the parent, which enables that organism to reproduce economically to itself; but, as we have already seen, when once the period of separation has become fixed, the number of young produced will be directly as the amount of nourishment obtained by the progenitor.
It is possible that amongst human beings scarcity of nourishment in a community, by favouring the early separation of the offspring, may give rise to a slightly greater number of births in the first generation; but any such increase is abundantly compensated by the extra mortality of such descendants, probably in the first months of life, certainly within the first year; and it may be held with safety that the number of the offspring which reach maturity, if the scarcity of nutrition continues, is only a fraction of the number that would become capable of multiplying under more favourable conditions. The idea that the birth-rate and, therefore, the mortality of the human race would decrease if food were abundant is a visionary hypothesis incapable of being held by any who have taken an enlarged view of the subject.
The law of reproduction is :—that, as dispersion is beneficial to an organism, every form tends to
produce young in far greater numbers than can possibly survive; these, by competition with each other and with surrounding circumstances, are improved by the survival of the fittest, whilst the number that survive is directly determined by the amount of food ; that although the principle of economy of nutrition causes the young to be separated from the parent at as early a period as is consistent with their chance of survival, when that period of separation becomes fixed the number of young produced in the first generation is in a direct ratio to the food supplied—although, when the period of separation varies, a larger number are sometimes produced by a temporary period of arrested nutrition, giving rise to an earlier separation of the young.
CERTAIN complicated phenomena connected with the process of development may be conveniently considered in this connexion.
Like all the phenomena of vitality, the period at which any organism commences to reproduce its species is subject to some slight variation. It is well known that the age of maturity varies much in the human race and domesticated animals. Many birds commence to breed before they have acquired their adult plumage *; and although it is difficult to obtain evidence, it is almost certain that the period at which the first nest is formed varies considerably in the same species, especially amongst sea-birds. Several amphibians breed whilst still retaining their larval characters; and the evidence that insects vary in their period of maturity is very strong, especially amongst the Hemiptera f. Myrmus miriformis, Fall., and Sphyracephalus ambulans, two British species of bug, are, as a rule, apterous,
* Cope, “ Origin of Genera,” Proc. Ac. Nat. Sci. Phil. 1868.
Ent. Soc. Trans. 1871, part ii. p. 195.
although winged females occasionally, but very rarely, occur ; and the apterous condition is accompanied by other signs of imperfection, especially in Myrmus, where the ocelli are extremely rudimentary, whilst they are largely developed in the winged individuals. It may be objected that in these cases we see the process by which wings are acquired, and that the undeveloped species are ancestral forms; this seems extremely improbable, as the species which exhibit an apterous condition belong to very different families. The sudden appearance of wings in a few individuals is more like reversion than a tendency to develop a new organ, especially as such wings are always like those of the nearly allied and normally winged species. According to Mr. Cope * the males of several amphipod crustaceans become sexually mature whilst still retaining their larval characters.
A remarkable instance of this kind occurs amongst the Echinodermata, if, as Müller thought, Echinocyamus, a form usually admitted as a distinct genus, be really sexually mature; for Agassiz has almost proved that it is a young form of some Clypeastroid f. Another example of early maturity occurs perhaps in the case of Echinus norvegicus,
* “ Origin of Genera,” Proc. Ac. Nat. Sci. Phil. 1868.