The object of this whole process of separation, first of the elements, then of the organs, then of the individuals, and last the increasing divergence of the individuals, is undoubtedly the funding of more and more diverse characters in a common offspring; and thus by increasing multiplicity of inheritance to insure larger variation in offspring and thereby furnish more abundant material for natural selection. This is far more than a compensation for the apparent wastefulness of this mode of reproduction.
If then the non-sexual preceded the sexual modes of reproduction, evidently, at first, only Lamarckian factors could operate. Evolution was then carried forward wholly by changes in the individual produced by the environment and by use and disuse of organs, continued and increased through successive generations indefinitely. It is probable therefore that for want of the selective factors, the rate of evolution was at first comparatively slow; unless indeed, as seems probable, the earliest forms were, as the lowest forms are now, more plastic under pressure of physical conditions than are the present higher forms. The great contrast between the Lamarckian and Darwinian factors in this regard, and the slowness of change now in higher forms under Lamarckian factors alone, is best shown in plants where either kind of factors may be used at pleasure. In these, if we wish to make varieties, we propagate by seeds—sexual reproduction—but if we wish to preserve varieties, we propagate by buds and cuttings—non-sexual reproduction.
We have taken the two Lamarckian factors together, in contrast with the Darwinian. But even in the two Lamarckian factors there is a great difference in grade. Undoubtedly the lowest and first introduced was pressure of the physical environment. For even use and disuse of organs implies some degree of volition and voluntary motion, and therefore already some advance in the scale of evolution.
With the introduction of sex another entirely different and higher factor was introduced, viz. natural selection, among a varying progeny, of the fittest individuals. We have already seen how sexual generation produces variation of offspring and how this furnishes material for natural selection. As soon, therefore, as this form of generation was evolved, this higher factor came into operation, and immediately, as it were, assumed control of evolution, and the previous factors became subordinate though still underlying, conditioning, modifying the higher. The result was an immediate increase in the speed and in the diversity of evolution. It is very worthy of note too, that it is in the higher animals, such as birds and mammals, where we find the highest form of sexual generation, where the diversity of funded characters and therefore the variation in the offspring is the greatest, and natural selection most active: it is precisely among these that the Lamarckian factors are most feeble, because during the most plastic portion of life the offspring is removed from the influence of the physical environment and from the effects of use and disuse, by their enclosure within the womb or within a large egg well supplied with nourishment. In these, development is already far advanced before Lamarckian factors can operate at all.
Next I suppose Physiological selection or Romanes's factor came into operation. After the introduction of sex, it became necessary, that the individuals of some varieties should be in some way isolated, so as to prevent the swamping of varietal characters as fast as formed, in a common stock by cross breeding. In very low forms with slow locomotion, such isolation might easily take place accidentally. Even in higher forms, changes in physical geography or accidental dispersion by winds and currents, would often produce geographical isolation; and thus by preventing crossing with the parent stock, secure the formation of new species from such isolated varieties. But in order to insure in all cases the preservation of commencing species, sexual isolation was introduced or evolved as I suppose later, and according to Romanes somewhat as follows:
All organs are subject to variation in offspring, but none are so sensitive in this regard as the reproductive organs; and these in no respect more than in relative fertility under different conditions. Suppose then the offspring of any parent to vary in many directions. By cross-breeding among themselves and with the parent stock, these are usually merged in a common type, their differences pooled, and the species remains fixed or else advances slowly by natural selection, along one line, as physical conditions change in geological time. But from time to time there arises a variation in the reproductive organs of some individuals, of such kind that these individuals are fertile among themselves, but sterile or less fertile with other varieties and with the parent stock. Such individuals are sexually isolated from others, or sexually segregated among themselves. Their varietal differences of all kinds are no longer swamped by cross-breeding, but go on to increase until they form a new species. It is evident then, as Romanes claims, that natural selection alone tends to monotypal evolution. Isolation of some sort seems necessary to polytypal evolution. The tree of evolution under the influence of natural selection alone grows palm-like from its terminal bud. Isolation was necessary to the starting of lateral buds, and thus for the profuse ramification which is its most conspicuous character.
Next, I suppose, was introduced, sexual selection, or contest among the males by battle or by display, for the possession of the female, the success of the strongest or the most attractive, and the perpetuation and increase of these superior qualities of strength or beauty in the next generation. This I suppose was later, because connected with a higher development of the psychical nature. This is especially true when beauty of color or song determines the selection. As might be supposed therefore, this factor is operative only among the highest animals, especially birds and mammals, and perhaps some insects.
Next and last, and only with the appearance of man, another entirely different and far higher factor was introduced, viz. conscious, voluntary co-operation in the work of evolution—a conscious voluntary effort to attain an Ideal. As already said, we call this a factor, but it is much more than a factor. It is another nature working in another world—the spiritual—and like physical nature using all factors, but in a new way and on a higher plane. In early stages man developed much as other animals, unconscious and careless whither he tended and therefore with little or no voluntary effort to attain a higher stage. But this voluntary factor, this striving toward a goal or ideal, in the individual and in the race, increased more and more until in civilised communities of modern times it has become by far the dominant factor. Reason, instead of physical nature, takes control, though still using the same factors.
Now, in this whole process, we observe two striking stages. The one is the introduction of Sex, the other the introduction of Reason.[69] They might be compared to two equally striking stages in the evolution of the individual, viz. the moment of fertilisation and the moment of birth. As the ontogenic evolution receives fresh impulse at the two moments of fertilisation and of birth; so the evolution of the organic kingdom at the two periods mentioned. With the appearance of sex, three new and higher factors are introduced, and these immediately assumed control and quickened the rate of evolution. With the appearance of reason in man another and far higher factor is introduced which in its turn assumes control, and not only again quickens the rate, but elevates the whole plane of evolution. This voluntary, rational factor not only assumes control itself, but transforms all other factors and uses them in a new way and for its own higher purposes.
[69] By Reason I mean the faculty of dealing with the phenomena of the inner world of consciousness and ideas, or reflection on the facts of consciousness. Animals live in one world, the outer world of sense; man in two worlds, in the outer world like animals, but also in the inner and higher world of ideas. All that is characteristic of man comes of this capacity of dealing with this inner world. In default of a better word I call it Reason. If any one can suggest a better word I will gladly adopt it.