Natural Selection as the Essential Factor in the Origin of Species
There are two great, universal, and very conspicuous characteristics of the whole organic world which, because they are so very common, were almost ignored before Darwin showed their importance. These are (1) the great variability in all common and widespread species, and (2) their enormous powers of increase.
The facts of variability are recorded in every book on Darwinism or on organic evolution, and it is only necessary here to appeal to the reader's own observation or to state a few illustrative facts. Everybody sees that among a hundred or a thousand people he knows or frequently meets no two are alike. This is variability. He also knows that the amount of the differences between them is often very large, and always, if you have any two of them side by side, easily perceptible and capable of being described. He also knows that they differ in every part and organ that can be seen: the height, the bulk of body; the shape of the hands, feet, head, ears, nose, and mouth; the proportions of the legs, arms, and body to each other; the
abundance and character of the hair—coarse or fine, straight or curly, and of all colours between flaxen and intense black. To declare that variability among men and women, even of the same race and in the same country, is a rare phenomenon, and that in amount it is infinitesimal, would be a ludicrous misstatement of the facts or a wilful perversion of the truth. But, as regards animals or plants in a state of nature, this misstatement has been made and has been used as an argument against the Darwinian theory. It is, however, now well known, as a matter of direct observation and measurement, that when a few scores or hundreds of individuals are compared, even in the same district and at the same season, they differ in their proportions to about the same amount, and to some extent in every visible part or organ, as do human beings.
This, however, was not well known when Darwin collected the materials for his various works, and he even sometimes makes the proviso—"if they vary, for without variation selection can do nothing"; and this has been taken as an admission that variation is a rare instead of being a universal phenomenon. He
also often spoke of the accumulation of small or minute variations, and this has led to the statement that variations are infinitesimal in amount, and therefore could, at first, be of no use to the possessor in the struggle for existence.
Rapid Increase of All Organisms
This is another fact of Nature which requires to be kept in mind in all discussions of the action of natural selection, yet it is often altogether ignored by critics of the theory. As an illustrative fact, a not uncommon European weed of the Cruciferæ family has been found to produce about 700,000 seeds on a single plant, whence it can be calculated that if every seed had room to grow for three successive years their produce would cover a space of about 2,000 times as large as the whole land surface of the globe. Some of the minute aquatic forms of life which increase by division in a few hours would, if they all had the means of living, in the same period occupy a space equal to that of the entire solar system. Even the largest and slowest breeding of all known mammals, i.e. the elephant, would, if
allowed space to live and breed freely for 750 years, result in no less than nineteen million animals.