THEIR GRADES AND THE ORDER OF THEIR INTRODUCTION.
The usually recognised factors of evolution are at least five; viz.: (1) Pressure of a changing environment affecting function and function affecting structure, and the changed structure and function inherited and integrated through successive generations indefinitely. (2) Use and disuse of organs reacting on growth-force and producing change in form, structure, and relative size of parts, and such change inherited and integrated through successive generations. (3) Natural selection among individuals of a varying progeny, of those most in accord with an ever-changing environment—or as it has been otherwise called "survival of the fittest" in each successive generation. (4) Sexual selection: the selection by the female, among varying male individuals all competing for her possession, of the strongest or the most attractive. Among mammals the selection is mainly of the strongest as decided by battle; among birds, of the most attractive as determined by splendor of color or beauty of song. (5) Physiological selection, or selection of those varieties, the individuals of which are fertile among themselves, but sterile or less fertile with other varieties and with the parent stock. This has also been called "segregate fecundity" by Gulick, and homogamy by Romanes.
These five factors are all usually but not universally recognised. The first two are Lamarckian, the second two Darwinian factors. In the Lamarckian factors the changes occur during individual life, and the offspring is supposed to inherit them unchanged. In the Darwinian factors on the contrary the changes are in the offspring, and the individuals during life are supposed to remain substantially unchanged. The fifth factor has, only very recently, been brought forward by Romanes and Gulick and is not yet universally recognised; but we believe that with perhaps some modifications it is certain to triumph. (6) To these recognised factors of organic evolution must now be added, in human evolution, another and far higher factor, viz. conscious, voluntary co-operation in the work of evolution, conscious striving for the betterment of the individual and of the race. This factor consists essentially in the formation and pursuit of ideals. We call this a factor, but it is also much more than a factor. It stands in place of nature herself—it is a higher-rational nature using all the factors of physical nature for its own higher purposes. To distinguish the evolution determined by this factor from organic evolution, we often call it progress.
Underlying all these factors as their necessary condition, and therefore themselves not called factors, are two opposite operative principles, viz. heredity and variability. Like the conservative and progressive elements in society, one tends to fixedness, the other to change. The one initiates change, the other accumulates its effects in successive generations. The one tries all things, the other holds fast to whatever is good. They are both equally necessary to the successful operation of any or all of the factors.
Let us now compare these six factors, as to their grade or position in the scale of energy and as to the order of their introduction.
The first two—Pressure of the environment and Use and disuse, i. e. the Lamarckian factors—are the lowest in position, most fundamental in importance, and therefore most universal in their operation. They are therefore also first in the order of time. They precede all other factors and were for a long time the only ones in operation. For observe: all the selective factors, viz. those of Darwin and Romanes, are wholly conditioned on Reproduction; for the changes in the case of these are not in the individual life but only in the offspring. And not only so but they are also strictly conditioned on sexual modes of reproduction. For all non-sexual modes of reproduction such as fission and budding are but slight modifications of the process of growth, and the resulting multitude of organisms may be regarded as in some sense only an extension of the first individual. There is thus a kind of immortality in these lowest protozoa. 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 or by use and disuse of organs—i. e. by 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 different characters, as it were offered for inheritance in every generation, some individuals will inherit more of one and some more of another character. In a word, sexual generation, by multiple inheritance, tends to variation of offspring and thus furnishes material for natural selection.
Thus then I repeat, all the selective factors are absolutely dependent for their operation upon sexual reproduction. But there was a time when this mode of reproduction did not exist. It is certain the non-sexual preceded the sexual modes of reproduction. I cannot stop now to give the reasons for believing this. I have already given them in some detail in a previous article[68] to which I would refer the reader. Suffice it to say now that the order of introduction of the various modes of reproduction culminating in the highest sexual modes is briefly as follows: (1) Fission. An organism of the lowest kind grows and divides into two. Each half grows to mature size and again divides; and so on indefinitely. In this case there is no distinction between parents and offspring. Each seems either or neither. (2) Budding. Growth-force concentrating in one part produces a bud, which continues to grow and individuate itself more and more until it separates as a distinct individual. This is a higher form than the last because in this case the individual is not sacrificed. Only a small part separates and the separated part is in some sense an offspring. We have therefore for the first time the distinction of parent and offspring. (3) By the law of differentiation and localisation of functions, the bud-forming function is next relegated to a special place and we now have a bud-forming organ. (4) By another general law, the law of interior transfer, the bud-forming organ is next transferred for greater safety to an interior surface and thus simulates an ovary, although not yet a true ovary or egg-forming organ. Examples of all these steps are found among existing animals.
[68] Genesis of Sex, Pop. Sci. Monthly, 1879, Vol. xvi. p. 167. Revue Scientifique, Feb. 14, 1880.
Thus far reproduction is non-sexual. But now comes the great step, i. e. the introduction of sexual reproduction, in its lowest forms. (5) This simulated ovary or bud-forming organ becomes a true ovary or egg-forming organ; or rather, at first, a combination of ovary and spermary. The same organ prepares two kinds of cells, male and female, germ-cell and sperm-cell, which by their union produce an egg which develops into an offspring; and not only an offspring in the sense of a separated part of a previous individual, but in some sense a new creature, the creation of a new individual. There is an enormous difference and even contrast between this and all preceding modes. In non-sexual modes one individual becomes two; in this, two individual cells unite to form one. It is an expensive, even wasteful mode unless attended with some great advantage. The nature of this advantage we will presently see.
Thus far we have given only the lowest form of sexual generation. The two sexual elements only, germ-cell and sperm-cell are separated from each other, but not yet even the sexual organs, ovary and spermary, much less the sexual individuals, male and female. (6) The sex-element-forming function is next differentiated and localised in two different organs, ovary and spermary, but not yet in two different individuals. This is hermaphroditism so common in plants and in lower animals. (7) The already separated sexual organs are next localised in different individuals, and we now have male and female individuals. This is the case in many plants and in all the higher animals. (8) And finally these male and female individuals become more and more diverse in character.