According to Darwin, it is to this correlation of parts that we must refer the variation of other parts besides the one intentionally altered in the course of breeding. It must be admitted that the mutual dependence of the parts plays a very important rôle in the economy and development of the animal body, as we shall see later, and these connexions still remain very mysterious to us. Especially is this the case with the connexion between the reproductive organs and the so-called secondary sexual characters. Removal of the reproductive organs or gonads induces, in Man, for instance, if it be effected in youth, the persistence of the childish voice and the non-development of the beard; in the stag the antlers do not appear, and in the cock the comb does not develop perfectly, &c., but we are not yet able to understand clearly why this should be so.
LECTURE III
THE DARWINIAN THEORY (continued)
Natural selection—Variation—Struggle for existence—Geometric ratio of rate of increase—Normal number and ratio of elimination in a species—Accidental causes of extinction—Dependence of the strength of a species on enemies—Struggle for existence between individuals of the same species—Natural selection affects all organs and stages—Summary.
In artificial selection, through which, with or without conscious intention, our domesticated animals and cultivated plants have arisen, there must obviously be three kinds of co-operative factors: first, the variability of the species; second, the capacity of the organism for transmitting its particular characters to its progeny; and third, the breeder who selects particular qualities for breeding. No one of the factors can be dispensed with; the breeder could effect nothing, were there not presented to him the variations of parts in the particular direction in which he wishes them to vary; an indefinite variation, that is, a variation not guided by selection, would never lead to the formation of new breeds; the species would probably become in time a motley mixture of all sorts of variations, but a breed with definite characters, transmissible in their purity to its descendants, could never be formed. Finally, every process of selective breeding would be futile, if the variations which appeared could not be transmitted.
Darwin assumes that processes of transformation quite similar to those which take place under the guidance of Man occur also in nature, and that it is mainly these which have brought about and guided the transformations of species which have taken place in the course of the earth's history. This process he calls natural selection.
It will readily be admitted that two out of the three factors necessary to a process of selective breeding are present also in the natural conditions of the life of species. Variability in some degree or other is absent from no species of animal or plant, though it may be greater in one than in another, and it cannot be doubted that the inborn differences which distinguish one individual from another are capable of transmission. It is only to untrained observers that all the individuals of a species appear alike; for instance, all garden whites, or all the individuals of the small tortoiseshell butterfly (Vanessa urticæ), or all the chaffinches. If the individuals are carefully compared it will be recognized that, even in these relatively constant species, no individual exactly resembles another; that even among butterflies twenty black scales may go to form a particular spot on the wings in one individual and thirty or twenty-five in others; that the length of the body, the legs, the antennæ, the proboscis exhibit minute differences; and it is probable that the same combination of quite similar parts never occurs twice. In many animals this cannot, of course, be proved, because our power of diagnosis is not fine enough to be able to estimate the differences directly, and because a comparison of measurements of all the parts in detail is not practicable. So we may here confine ourselves to the differences in the human race, which we can recognize with ease and certainty. Even as regards the face alone, all men differ from one another, and, numerous and complete as likenesses may be, it is impossible to find two human beings in which even the characters of the face are exactly similar. Even so-called 'identical twins' can always be distinguished if they are directly compared either in person or in a photograph, and if the rest of the body be also taken into consideration we find numerous small, sometimes even measurable differences.
The same is true of animals, and it is only our lack of practice that is at fault if we frequently fail to detect their individual differences. The Bohemian shepherds are said to know personally, and be able to distinguish from all the rest, every sheep in their herds of many thousands. Thus the factors of variability and transmissibility must be granted, and it remains only to ask: Who plays the part of selecting breeder in wild nature? The answer to this question forms the kernel to the whole Darwinian theory, which ascribes this rôle to the conditions of life, to definite relations of individuals to the external influences which they meet with during the course of their lives, and which together make up their 'struggle for existence.'