In conclusion, let me again impress upon the reader that all the doubt and discussion, above described, as to the factors of evolution, is entirely aside from the truth of evolution itself, concerning which there is no difference of opinion among thinkers.
CHAPTER III.
THE GRADES OF THE FACTORS OF EVOLUTION AND THE ORDER OF THEIR APPEARANCE.
We have given in the previous chapter six factors of evolution—viz.: 1. Pressure of the environment. 2. Use and disuse of parts. 3. Natural selection. 4. Sexual selection. 5. Physiological selection. 6. Reason. Let us now compare these as to their grade in the scale of energy and as to the order of their introduction.
The first two or the Lamarckian factors are the lowest in position, the most fundamental and universal, and therefore the first in the order of appearance. They precede all other factors, and were doubtless for a long time the only ones in operation. For, observe, all the selective factors—i. e., those of Darwin and Romanes—are conditioned on reproduction; for the changes produced by these are not in the individual during life, but in the offspring at birth. And not only so, but the operations of these factors are further conditioned on sexual modes of reproduction; for all the non-sexual modes of reproduction—as, for example, by fissure and by budding—are but slight modifications of growth, and the resulting multitude of organisms may be regarded as in some sense only an extension of the first individual. Of course, therefore, the identical characters of the first individual are continued indefinitely, except in so far as they are modified in successive generations by the effect of the environment and by use and disuse—i. e., by the Lamarckian factors. In sexual generation, on the contrary, the characters of two diverse individuals are funded in a common offspring; and the same continuing through successive generations, it is evident that the inheritance in each individual offspring is infinitely multiple. Now, the tendency to variation in offspring is in proportion to the multiplicity of the inheritance: for among the infinite number of slightly differing characters, as it were, offered for inheritance in each generation, some individuals will inherit more of one and some more of another character. In a word, sexual reproduction by multiple inheritance tends to variation of offspring, and thus furnishes material for natural selection.[16]
Thus, then, I repeat, all the selective factors are absolutely dependent on sexual modes of reproduction. But there was a time when this mode of reproduction did not yet exist.[17] The sexual modes developed out of non-sexual modes. If these non-sexual preceded sexual modes of reproduction, it is evident that at first only Lamarckian factors could operate. Evolution was then carried forward wholly by changes in the individual produced by environment and by use and disuse (acquired characters), inherited and increased by integration through successive generations indefinitely. It is probable, therefore, that the rate of evolution was at first comparatively slow; unless, indeed, as seems probable, the earliest forms were then and the lowest forms are now more plastic under the influence of physical conditions than are the present higher forms. Doubtless, now, in the higher animals and plants, the Darwinian factors are by far the most potent; for, among plants, where we can use these factors separately, 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).
I have taken the two Lamarckian factors together, and showed that they preceded the Darwinian. But even in the two Lamarckian factors there is a difference in grade. Undoubtedly the lowest, the most fundamental, and therefore the first introduced, was pressure of the physical environment. For 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, or selection of the fittest individuals of a varying progeny. We have already seen how sexual generation produces variation of offspring, and how this furnishes materials for natural selection. As soon, therefore, as this form of generation was evolved, this higher factor came into operation and immediately assumed control; while the previous factors became subordinate, though still underlying, conditioning, and modifying the activity of the higher. The result was an immediate increase in the rate of evolution. It is very worthy of note that it is in the higher animals, such as birds and mammals, in which we have only the highest forms of sexual reproduction, where the diversity of characters of the two sexes funded in the offspring is the greatest, and where, therefore, the variation in offspring is also greatest and natural selection most active; it is precisely among these that the Lamarckian factors are most feeble, because, during the most plastic period of life, the offspring is removed from the influence of the physical environment, and from use and disuse by its inclosure within the womb, or within a large egg surrounded with abundant nutriment. Development is already well 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 isolated in some way, 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, or partial or complete infertility of some varieties with other varieties and with the parent stock, was introduced, as I suppose, later. The process by which this takes place has already been explained. According to Romanes, natural selection alone, with cross-breeding, tends to monotypal evolution; isolation of some kind is necessary for polytypal evolution. The tree of evolution, under the influence of natural selection alone, grows, palm-like, from its terminal bud; isolation of varieties was necessary for the starting of lateral buds, and thus for the profuse ramification which is its most conspicuous character.