Sterility and conception / by Charles Gardner Child.

  • Child, Charles Gardner, 1873-1941.
Date:
1931
Catalogue details

Licence: In copyright

Credit: Sterility and conception / by Charles Gardner Child. Source: Wellcome Collection.

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    older variety would almost completely disappear. There are many instances known of such replacements occurring in primitive nature, and it is quite evident that the rate of replacement of the old by the new would depend in great part on the fertility of the new. It has many tirties been observed that an intense elimination of individuals, unaccompanied by life-saving peculiarities or variations, will move a species in a certain direction which is known as lethal selection, but it is likewise plain that variants that possess some valuable life-saving quality will predominate with greater rapidity if they are at the same time more prolific than their neighbors, and such instances are known as reproductive selection. If only a casual review of plant and animal life be made, we are at once impressed with the fact that they are extraordinarily fertile and it would appear oftentimes extravagantly so. It has been estimated that if all the progeny of but one oyster survived and multi¬ plied “its great, great, great, grand offspring would number sixty-six with thirty-three noughts after it and the heap of shells would be eight times the size of our earth.” When a family reaches into the thousands or millions, there is a wide margin to allow for accidents, so that careful nursing is of minor importance; and in the lower forms of life, where parent rearing of such offspring is impossible, the spawning method has obvious advantages. From the point of view of the parent advantage, there are manifest disadvantages inherent in this method. Multitudinous production often causes the exhaustion and even the death of the mother, as is instanced not only in the delicate butterfly, but in the strong marine lamprey, where she dies in the process of reproduction. There is an old-world type, the peripatus, that, although having no armor nor weapons for offense or defense, has held its own in many parts of the world for millions of years. It has been able to do this because of its nocturnal and elusive habits, and because its young are carried by the mother for a long time before birth, and when finally born appear as miniature adults, ready at once to provide for them¬ selves. Almost all animals, with the exception of man, have a regular breeding season which is devoted to the propagation of their species. 1 his period of sexual activity is to a considerable extent modified by climatic conditions and environment, any marked change in the life habits of an animal, as Darwin has shown, tends to affect its powers of reproduction, and many animals breed poorly, or not at all, in cap-