But while I believe that sexual selection in particular has had much to do with producing the colours of Lepidoptera, the basis of all these colour variations must still be looked for in germinal selection, and we shall see later on how it is possible to think of the diversified and often relatively abrupt transformations of marking as the resultant of the co-operation of climatic influences with germinal selection.
Of course there must also be unimportant changes in butterfly-markings which depend solely on the internal play of forces in the determinant system, and to this must be referred the markings of many of the 'variable' species whose variations are mere fluctuations in the details of marking, which have therefore caused much trouble to the systematists. Truly unimportant variations will rarely or never combine into a 'constant' form, and the fact that there are species which are 'variable' in such a high degree is enough to make us refer their variations to their lack of importance, for if they possessed any biological value the less valuable among them would gradually be removed by selection. Perhaps the variable species of certain moths like Arctia caja, and especially Arctia plantaginis, the little 'bear' of the Alps and Apennines, must be reckoned among these. But from the fact that there are such fluctuations in the markings of Lepidoptera, it seems to me that we must conclude that species which show a high degree of constancy in their markings have been influenced by selection, or by climatic influences which turned the play of forces within the determinant system in the same direction in all individuals. All these considerations and conclusions are quite sound and serviceable theoretically, but they are difficult to apply to individual cases, and where this is attempted it must be with the greatest caution, and, if possible, on a basis of investigations specially undertaken for the purpose; for how should we know whether a species which to-day is highly variable may not a geological epoch later become a very constant one? We must in any case assume that marked fluctuations of characters are associated with many transformations.
LECTURE XXVI
GERMINAL SELECTION (continued)
Germinal selection, spontaneous and induced—Climatic forms of Polyommatus phlæas—Deformities—Excessive augmentation of variations—Can it lead to the elimination of a species?—Saltatory variations, copper-beech, weeping trees—Origin of sexual distinguishing characters—Formation of breeds among domesticated animals—Degenerate jaws—Human teeth—Short-sightedness—Milk-glands—Small hands and feet—Ascending variation—Talents, intellect—Combination of mental endowments—The ultimate roots of heritable variation—There are only plus- and minus-variations—Relations of the determinants to their determinates—The play of forces in the determinant system of the id—Germinal selection inhibited by personal selection—Objection on the score of the minuteness of the substance of the germ-plasm.
Hitherto we have derived the variations of the determinants of the germ-plasm, upon which we based the process of germinal selection, from chance local fluctuations in nutrition, such as must occur in an individual id, independently of the nutrition of the other ids of the same germ-plasm. But there are doubtless also influences which set up similar nutritive changes in all ids, and by which, therefore, all homologous determinants, in as far as they are sensitive to the nutritive change in question, are affected in the same manner. To this category belong changes in the external conditions of life, and particularly climatic changes. It is, then, germinal selection alone which brings about the presence of a majority of ids with determinants varying in the same direction, and personal selection has no part in the transformation of the species. Many years ago I instituted experiments with a small butterfly, Pararga egeria, and these showed that a heightened temperature so influenced the pupæ of this form that the butterflies emerged with a different and deeper yellow ground-colour, similar to that of the long-known southern variety Meione. More thoroughly decisive, however, were the experiments on Polyommatus phlæas, the small 'fire-butterfly,' which were carried on in the eighties by Merrifield in England and by myself almost at the same time. I shall discuss these later in more detail, and will only say here that this butterfly, whose range extends from Lapland to Sicily, occurs in two forms, the southern distinguished by a 'dusting' of deep black from the northern, in which the wing-surfaces are of a pure red-gold. The experiments showed that the southern form can be artificially produced by warmth, and the interpretation must be that the direct influence of higher temperature affects the quality of the nutritive fluids in the germ-plasm, and thereby at the same time the determinants of one or more kinds of wing-scales are caused to vary in all the ids in the same direction, in such a fashion that they give rise to black scales instead of the former red-gold ones. It is thus certain that there are external influences which cause particular determinants to vary in a particular manner. I call this form of germinal variation 'induced' germinal selection, and contrast it with 'spontaneous' selection, which is caused, not by extra-germinal influences, but by the chances of the intra-germinal nutritive conditions, and which will, therefore, not readily occur at the same time in all the ids of a germ-plasm, and so will not give rise to variation of the same kind in the homologous determinants of all the ids.
The two processes must also be distinguished from each other in their relation to personal selection, for induced germinal selection will go on increasing until the maximum of variation corresponding to the nature of the external influences and of the determinants concerned is reached. Since all the ids are equally affected and caused to vary in the same way, personal selection has nothing to take hold of, and the variation might go on intensifying even if it should become biologically prejudicial. But it is quite otherwise with spontaneous germinal selection, which has its roots not in all, but only in a majority of the ids. Here the variation may go on increasing by germinal selection alone, but only until it acquires a positive or negative biological value, that is, until it becomes advantageous or prejudicial to the life of the individual; then personal selection intervenes and decides whether it is to go on increasing or not. Spontaneous germinal selection can therefore only lead to the general variation of a whole species when it is supplemented by some external factor such as, especially, the utility of the variation.
This does not imply, however, that indifferent variations of large amount could not arise through spontaneous germinal selection, but they would remain confined to a small number of individuals, and would sooner or later disappear again. The congenital deformities of Man may in part fall under this category. If, for instance, certain determinants are, by reason of specially favourable local nutritive conditions, maintained for a long time in progressive variation, they will become so strong that the part which they determine will turn out excessive, perhaps double. Hereditary polydactylism in Man may perhaps be explained on this principle, and I had already referred it to the more rapid growth and duplication of certain determinants of the germ even before formulating the idea of germinal selection. In this I was at one with the pathologist Ernst Ziegler, who had designated polydactylism as a germ-variation, and in contrast to others had not interpreted it in an atavistic sense, as a reversion to unknown six-fingered ancestors. All excessive or defective hereditary malformations may be referred to germinal selection alone, that is, to the long-continued progressive or regressive variation of particular determinant-groups in a majority of ids.