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stability on the earth. Animal and vegetable life, in the present geological age, is thought to be in a state of much less rapid change than characterized it during earlier times; and it is not unlikely that this is due to the present slow rate and small amount of change in the conditions of life.
Let us place ourselves, then, in a position to realize as fully as possible the great effect upon transmutation of species that must have been produced by the world-wide and profound alterations in the earth's surface which are known to have taken place through past ages. Let us remember how inconceivably long those ages were in comparison with any periods of time during which scientific observations have been made by man, and we shall, I think, feel, that, in this brief period, naturalists have found change enough going on to justify the inference that all species, from the beginning, may well owe their existence to similar causes with those just mentioned, vastly greater in amount, and acting through stupendous periods of time. How variations of species, once having been produced by such causes as those just described, have been preserved and intensified will be explained in the next chapter.
CAUSES WHICH HAVE PRESERVED ADVANTA
STRUGGLE for existence inevitably follows from the high rate at which all organic beings tend to
increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. . . . Linnæus has calculated, that if an annual plant produced only two seeds — and there is no plant so unproductive as this — and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase. It will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old : if this be
after period of from seven hundred forty to seven hundred fifty years, there would be nearly nineteen million elephants alive, descended from the first pair.”i
1 Origin of Species, pp. 50, 51.
In introducing the subject of the present chapter, I quote these few sentences literally from Mr. Darwin ; because he has done so much more than any other investigator to call the attention of the scientific world to these facts, which, up to the time of his researches and publications on the subject, had, for the most part, passed unobserved, or at any rate unregarded. And so it has become almost impossible, even in the simplest statement of facts concerning the rate of increase of living beings in its relation to the origin of species, to avoid giving the substance, if not even the exact language, of Mr. Darwin.
In the two estimates just quoted, a plant and an animal with an exceptionally slow rate of increase were purposely chosen. As an example of the opposite kind, the codfish may be taken. At one spawning it produces from four to nine million eggs, each of which, if allowed to hatch, would in the course of a few years grow into a fish of from five to twenty pounds' weight. It is easy enough to see, that if all the eggs hatched, and all the young cod grew up, their natural rate of increase would very soon pack the waters of the ocean brimful of fish. More rapid still is the rate of multiplication of the little microscopic plants (Bacteria), which are now generally recognized as the chief cause of decay or putrefaction. So rapidly do they increase, that for the offspring of a single individual (itself invisible to the naked eye) to fill the waters of the ocean would take at most but about five days."
A striking instance of the rapidity with which, under favorable circumstances, animals have actually spread, is given by Mr. Darwin in his account of the rabbits on the Island of Porto Santo, near Madeira.? These were introduced by a female rabbit with young being left on the island in 1418 or 1419 by the crew of a passing ship; and they soon became so abundant as to force the settlers on the island to abandon it. Their rapid increase may be attributed to the fact that no birds of prey, or other animals capable of destroying the rabbits, existed on the island; and the climate, as well as the supply of foud, must have been especially well adapted to their wants.
It is interesting to notice that these rabbits have changed so much (becoming nearly three inches shorter, and almost one-half less in weight, than wild English rabbits, besides undergoing considerable changes in color), that they would not now, if found in the wild state with their history unknown, be ranked as of the same species with the English rabbit. And the naturalist would be still less likely to consider them as identical with the latter species (which is the same as the Spanish ancestor of the Porto Santo breed) from the fact that they will not cross with the wild English rabbit. It is well known how soon
i Cohn, quoted by Burrill, The Bacteria, p. 9. ? Animals and Plants, etc., vol. i. pp. 117-120.
new breeds of domestic cattle, sheep, or hogs, if found desirable, become prevalent over wide regions; so that it was said in England, that the introduction of short-horned cattle operated almost like a pestilence in the destruction of the earlier and less improved breeds. Of course, the comparison to a pestilence means only this, – that the owners of the older sorts killed them off, or disposed of them to the butcher, at such a rate as to thin them out as fast as the cattle-plague could have done it.
The same holds good in regard to the introduction of desirable new agricultural plants, as, for instance, in the case of the Early Rose potato. Seeds, tubers, bulbs, or cuttings of new varieties, will usually, in the course of four or five years from the time of their introduction, be so common as no longer to command a price much above that of older varieties. Similar to the spread of the Porto Santo rab