It might be objected that, through such a continual weeding-out of rebellious determinants, the germ-plasm would become so constant in its constitution that it would ultimately be secure from all such aberrations of it on the part of its determinants, and therefore would in time become quite incapable of diverging from its proper path at all, and would thus no longer require this continual correction through amphimixis.
I do not wish to contradict this conclusion; indeed, I believe that the constitution of the species becomes more and more constant in the way I have indicated, and that an ever more perfect and stable equilibrium of the whole determinant system is thus brought about. It follows that in the course of generations the diverse determinants of the germ-plasm will vary within a progressively shortened radius, and will thus more and more rarely overstep the limits of the 'variation-playground'—and yet I still believe that this justifiable conclusion tells in favour of my interpretation of the utility of the persistence of amphigony once introduced.
Let it be remarked, in the first place, that it is by no means essential to the preservation of a useful institution that it should practically justify its utility in every generation. Although, for instance, the warm winter coat of a species of mammal may be necessary to its survival, it does not disappear at once when a winter happens to occur which is so warm that even individuals with poor pellage can survive. Indeed, several such mild winters might occur in succession, in which there was no weeding-out of the individuals with poor fur, and yet the thickness of the winter fur of the species would not become less fixed, just because this character no longer varies perceptibly in an old-established species which has long been perfectly adapted, and it could only be brought into a state of marked fluctuation very slowly through direct influence on the germ-plasm, or through panmixia. But exactly the same thing is true in regard to the determinants of the reproductive cells, in respect of their adaptation to amphimixis, only very much more emphatically.
Before going further, I should like to show that the conclusion we have just deduced from the theory, namely, that the equilibrium of the determinant system of a species increases in stability with the duration of its persistence, holds good not only for the whole system, but for its individual parts, that is, for the individual characters and adaptations. Experience teaches that characters are the more exactly and constantly transmitted the older they are; generic characters are more constant than species-characters, order-characters more persistent than family-characters—this is implied even in their name. But we are able to show even in relation to the characters of a species that those which have been fixed for a long time are most precisely and purely transmitted; that is, that their determinants are least inclined to overstep the limits of the 'variation-playground' either in an upward or downward direction.
Two groups of facts prove this: first the observed fact that the very different degree of variability which the different species exhibit is by no means common to all the characters of the species in the same measure; for individual characters may be variable or constant in very different degrees.
Many years ago[21] I drew attention to the fact that the different stages in the life-history of insects, especially of Lepidoptera, might be variable in quite different degrees. Thus, for instance, the caterpillar might be very variable, and yet the butterfly which arises from it might be extremely constant. I concluded from this—what probably no one now will dispute—that the various stages may vary phyletically independently of one another, that, for instance, the caterpillar may adapt itself to a new manner of life, a new food-plant, a new means of defence, while the butterfly, unaffected by this, goes on quietly as it was before. Every new adaptation necessarily implies variability, and so the stage which is in process of transformation must have its period of variability, which gradually returns again to greater constancy, and this the more completely the longer the series of generations through which the weeding out of the less well-adapted has endured.
[21] Studien zur Descendenztheorie, Leipzig, 1876.
But it is not only the individual stages of development that may be unequally variable; the same is true of the characters of a species which occur simultaneously. The most striking example of this known to me is the leaf-butterfly, which I have already mentioned many times in the course of these lectures—the Indian Kallima paralecta. In this species the brown and red upper surface is almost alike in colour and marking in all individuals, but the under surface, the colour and marking of which is so deceptively mimetic of a leaf, is variable to such a degree that it is difficult, among a large number of specimens, to find even a few which are as like one another as are the members of species in which the under side is constant. It need not be urged that this is due to the complexity of the marking on the under side. In many of our indigenous butterflies the under side is just as complex in coloration and marking, and nevertheless it is very constant, being almost identical in all individuals, as for instance in Vanessa cardui. In Kallima the great variability of the under surface certainly depends not merely on the fact that the mimetic character has been only recently acquired (phyletically speaking), but chiefly on the fact that the dead leaves to which they approximate are themselves very diverse in appearance, for many are dry, others moist and covered with mould, and that the adaptations have therefore gone in different directions, and as yet, at least, have neither combined to form a single constant type, nor diverged to form two or three distinct types. The various 'leaf-pictures' seem equally effective in concealing the insects from their enemies, and thus there is still a continual crossing and mingling of the different essays at leaf-picturing.
A second group of facts, which indicates that old-established characters have less tendency to overstep the limits of the neutral 'variation-playground,' is to be found in the experience of breeders, and especially that of gardeners who have brought wild plants under cultivation in order to procure varieties.
It has been proved that the wild plants often exhibit no hereditary variations for a long series of generations, notwithstanding the greatly altered conditions of life, but that then a moment comes in which isolated variations crop up, which may then be intensified by the manipulations of the breeder to form sport-species with large conspicuously coloured flowers, or with some other distinctive character. Darwin called this a shattering of the constitution of the plant; but the stable and slowly varying 'constitution' simply means that in old-established and well-adapted species the determinants possess only a very restricted 'variation-playground,' and because of their firmly based harmonious correlation are not easily and never very quickly induced to overstep its limits in any marked degree.