We may perhaps give a preliminary statement of this law as follows: It is impossible that any part or organ should be removed suddenly from the ontogeny without bringing the whole into disorder, and the least serious disturbance of the course of development will undoubtedly be caused if the final stage of the part in question become rudimentary first. Only after this has happened, and the neighbouring parts have adapted themselves to the disappearance, can this extend to the stages immediately preceding it, so that these too degenerate, and allow the surrounding parts to adapt themselves. The further back into the ontogeny the disappearance extends the greater will be the number of other structures affected in some way or other by the degeneration, and these must not all be brought suddenly into new conditions, else the whole course of development would suffer. Thus at first only those determinants may disappear—and can disappear according to the laws of germinal selection—which control the final form of the useless organ, then those just preceding them, which controlled, let us say, its size, and thus more and more of the previously active determinants disappear, and hand in hand with this disappearance there is variation of all the parts correlated with the dwindling condition of the organ, so that their own development and that of the animal as a whole suffers no injury. If it were otherwise, if when a part became useless its collective determinants were all to disappear at the same time, the whole ontogeny would totter, in fact it would be much as if a man who wished to remove the breadth of a window from a house standing on pillars were to begin by taking away the foundation pillar.
It is, of course, to be understood that these processes go on so exceedingly slowly that personal selection takes a share in them, at least at the beginning. Later on, the further degeneration of a useless organ or rudiment has no effect on the individual's power of life, and therefore depends solely upon the struggle of the parts within the germ-plasm (germinal selection).
If we could see the determinants, and recognize directly their arrangement in the germ-plasm and their importance in ontogeny, we should doubtless understand many of the phenomena of ontogeny and their relation to phylogeny which must otherwise remain a riddle, or demand accessory hypotheses for their interpretation. Several years ago Emery rightly pointed out that the phenomena of the variation of homologous parts might be inferred by reasoning from the germ-plasm theory. If one hand has six fingers instead of five, it not infrequently happens that the other also exhibits a superfluity of fingers, and sometimes the foot does so too. The phyletic modification of the limbs in the Ungulates has taken place with striking uniformity in the fore and hind extremities; no animal has ever been one-hoofed in front and two-hoofed behind. Although I might suggest that this primarily depends on adaptation to different conditions of the ground, and that the Artiodactyls were evolved in relation to the soft marshy soil of the forest, and the Perissodactyls for the steppes, it cannot be denied that germinal conditions may have co-operated in bringing about this uniformity of the direction of variation, especially as the whole structure of the fore- and hind-limbs exhibits such marked similarity. Emery is inclined to refer this to 'germ-plasmic correlations,' and we have assumed from the very first that the different determinants and groups of determinants do indeed stand in definite and close relations to one another. But it seems to me premature to say anything more precise and definite than that in the meantime. I should like, however, to say that determinants or groups of determinants which had in old ancestral germ-plasms to give rise to a series of quite similar structures by multiplication during the ontogeny, and therefore only needed to be present singly in the germ-plasm, would, in later descendants, have to shift their multiplication back into the germ-plasm itself, if necessity required that the homologous parts which they controlled should become different from each other. Then the previously single group of determinants in the germ-plasm would have to become multiple. But as new determinants can only arise from those which already exist, these new ones must have had their place beside the old, and would therefore probably be exposed to any intra-germinal causes of variation in common with them—that is to say, they will tend to vary even later in a similar manner. For instance, we might think of the segments of primitive Annelids, which in form and contents are for the most part alike, as arising from one germ-rudiment, from which, when, in the higher Annelids, the various regions of the body had to take a different form, several primary constituents of the germ-plasm separated themselves off; and in a similar way the much higher and more complex differentiation of the somatic segments in the Crustaceans must have been brought about. Thus we understand how the determinant groups of the germ-plasm multiplied according to the need for increasing differentiation, but remained in intimate relation, which exposed them in some measure to a common fate, that is, to common modifying influences, and in many cases determined them to similar variation.
But we cannot see directly into the germ-plasm, and are therefore thrown back on the inductions we can make from the facts presented to us by the phenomena of visible living organisms. As yet the material for such inductions is scanty, because it has been got together haphazard, and not collected on a definite plan. I therefore refrain for the present from attempting any further elaboration of my germ-plasm theory. It is only when an abundance of observation material, collected according to a definite plan, lies at our disposal that anything more in regard to the intimate structure of the germ-plasm, or the mutual influences and relations of its determinants and its modification in the course of phylogeny can be deduced with any certainty. Meanwhile, we must content ourselves with having, through the hypothesis of determinants, made intelligible at least the one fundamental fact, how it is possible that in the course of the phylogeny single parts and single stages can be thrown out or interpolated, or even only caused to vary, without giving rise to variation in all the rest of the parts and stages of the animal. A theory of epigenesis cannot do this, for, if no representative particles were contained in the germ-plasm, then every variation of it would affect the whole course of development and every part of the organism, and variations of individual parts arising from the germ would be impossible.
LECTURE XXVIII
THE GENERAL SIGNIFICANCE OF AMPHIMIXIS
Twofold import of amphimixis—It conditions the continual changing of individuality—Analogy from game of cards—The germ-plasm is at once variable and persistent—The two roots of individual variation: germinal selection and new combinations of the ids—'Harmonious' adaptation conditions amphimixis—Difference between adaptation and mere variation—Is a 'direct' use of amphimixis to be insisted upon?—Ceaseless intervention of personal selection in the lineage of the germ-plasm—Far-reaching effects of personal selection—Fixing of the arrangements for amphimixis in the course of generations of species—Increase of the constancy of a character with its duration—Characters in the same species variable in different degrees—The upper and under surfaces of Kallima—Wild plants brought under cultivation do not at first vary—Amphimixis very ancient, therefore very firmly established—Does amphimixis bring about equalization (Hatschek, Haycraft, Quetelet)?—Galton's frequency curves—Ammon's free scope for variations—De Vries' asymetrical curves of frequency.
We have already made ourselves familiar with the process which in unicellular organisms is called conjugation and in multicellular organisms fertilization, and we have seen that its most obvious significance lay in the fact that through it the germ-plasms of two individuals are united. Since, according to our view, this germ-plasm or idioplasm is the bearer of the hereditary tendencies of the organism concerned, the mingling or amphimixis of two germ-plasms brings together the hereditary tendencies of two individuals, and the organism whose development is derived from this mingled germ-plasm must therefore exhibit traits of both parents, and must to a certain extent be made up of the traits of both. This is one result attained by amphimixis.