Of course it is possible that the mutual adaptation of the Algæ and Fungi in the evolution of species of Lichens took place very long ago, at a time when sexual reproduction still existed, at least in one of the associated organisms, the Fungus. The Ascomycetes, to which most of the Lichens belong, do not at present usually exhibit the process of amphimixis, as I have already noted; but it may perhaps be still possible to decide whether they must have exhibited it, or at least could have exhibited it at an earlier stage in their evolution. As the group of Thallophytes is a very ancient one, it is not inconceivable that the modern species of Lichens have existed for a long time, and that they had their origin in the remote past with the assistance of amphimixis.
Nor need it be objected to this supposition that it has been found possible to make new Lichens by bringing together Fungi and Algæ which had not previously been associated with one another; for in the first place both were already adapted to partnership with other species, and, moreover, so far no one has succeeded in rearing these artificial lichens for any length of time, still less in seeing them evolve into specific forms persistent in natural conditions.
But if this supposition should prove to be not only improbable, but actually erroneous, then the existence of Lichens would afford a clear proof that the 'type' of the species does not depend essentially upon the constant intermingling of individuals, but upon a process which we may best designate uniformity of adaptation. We have simply to suppose that under similar external influences similar variational tendencies were started by germinal selection in all the individuals of the two parent species of a lichen, and set a-going by germinal selection, just as a warmer climate gives rise to a black variety in the butterfly Polyommatus phlæas, because similar determinants of the germ-plasm of all the individuals were impelled to vary in the same manner and direction. This would then give rise to quite definite variations, and since only the suitable variational tendencies could survive, primitive though never complicated adaptations would arise. But we cannot assume that the lichens are not adapted to the conditions of their life as well as all other organisms. We cannot judge how far even their shape is to be regarded as an adaptation, whether the formation of encrusting growths, of tree-like forms, of cup or bush-lichens, may not be regarded as adaptations towards a full utilization of the conditions of their life—but even if this is not the case, the formation of soredia remains an undoubted adaptation to the symbiosis of those lichens which exhibit them. The soredia cannot depend upon the direct effect of the conditions of life, for they are reproductive bodies which did not exist before the existence of the lichen, and only originated to facilitate their distribution.
Thus there is still a great deal that is doubtful in our theories as to the transformations of organisms, and much remains still to be done. But even though we may doubt whether adaptations could come about in multicellular organisms without amphigony, we may be quite certain of the converse, that is, that the specific type can be changed in every individual feature by natural selection on the basis of amphigony, even as regards invisible features which only express themselves in altered periods of growth. Even when there is no isolation whatever and no mutual sterility, and when a mobile species is uniformly distributed over a large area, a splitting up into races in regard to one particular character may occur, simply through adaptation to the spatially different climatic conditions of the area inhabited.
Early in these lectures we discussed the twofold protective value of the coloration of the 'variable hare' (Lepus variabilis), which is distributed over the Arctic zone of the Old and New World, and also occurs in the higher regions of the Alps. Wherever there is a sharp contrast between winter and summer the variable hare exhibits the same specific type, being brown in summer and white in winter, but in regard to this very character of colour-change it forms races to some extent, for it is white for a longer or shorter time according to the length of the winter—in Greenland for the whole twelve months of the year, in Northern Norway only for eight or nine months, in the Alps for six or seven months, but in the south of Sweden and in Ireland not at all. There it remains brown in winter like our common hare (Lepus timidus). This is not a question of the direct effect of cold; if it were the species would become white in Southern Sweden also, for there is no lack of severe cold there, but the ground is not so uninterruptedly covered with snow, and so the white colour of the hare would be as often, probably oftener, a danger than a safeguard, and the more primitive double coloration has therefore been done away with by natural selection. The change of colour is thus hereditarily fixed, as is proved by the fact that the Alpine hare, if caught and kept in the valleys below, puts on a white dress at the usual time, which the common hare never does.
As in Southern Sweden the winter coloration has been wholly eliminated, so, conversely, from there to the Arctic zone the summer colouring has been more and more crowded out, and in the Farthest North it has totally disappeared from the characters of the species. We thus see that wherever the species lives the double colouring is regulated, as regards the duration of the winter coat, in exact harmony with the external conditions. There is a pure white, a pure brown, and a colour-changing race, and the latter is subdivided into two—one wearing the winter dress for six, the other for eight months. Probably these could be still further subdivided, if the different regions of the Scandinavian Peninsula were investigated individually from south to north. That the duration of the winter dress has its roots in the germ-plasm, and does not depend solely on the earlier or later period at which the cold sets in, is made clear by the two extreme forms, the white and the brown Lepus variabilis, as well as by the behaviour of captive animals. The familiar case of Ross's lemming, which remained brown in the warm cabin, and then suddenly became white when it was exposed to the cold of winter, only shows that the cold acts as a liberating stimulus. The preparatory changes in the pellage are already present, and the stimulus of cold brings them rapidly to a climax. Here, therefore, the necessary variations of the relevant germinal parts must have continually presented themselves for selection, which is intelligible enough, since it is merely a question of plus- or minus-variations. The fact that the six-months' dress can be transformed into an eight-months' dress must have its cause in some minute biological units of the germ-plasm; the determinants of the fur must be able to vary in such a way that a longer or shorter duration of the winter's coat is the result. The possibility of the whole variation depends upon the continual fluctuations of all determinants, now towards plus, now towards minus, and the necessity and inevitableness of each adaptation to the duration of the winter lies in the unceasing personal selection—the inexorable preferring of the better adapted.