In the case of the sequoias, we know something also of the history of their migrations. In Miocene times they were abundant, and of many species in circumpolar regions. Some twenty-four species of fossil sequoias are known, fourteen of which are Tertiary. By the cold of the Glacial epoch they were driven slowly southward, both in America and in Europe—in America as far as Southern California. After the Glacial epoch, and the return of temperate conditions, they doubtless attempted to go northward again; but these great changes were too much for them; they were wholly exterminated in Europe, and nearly so in America. A few were left stranded high up on the slopes of the Sierra Nevada, and on the cool, moist slopes of the Coast Ranges. The species now in California are not identical with those found in the Miocene strata of Greenland; but the difference is only what we might expect after such extensive migrations and such long and severe struggle for life. Further, it is noteworthy that the Miocene species fall into two groups, viz., the yew-like leaved and the cypress-like leaved. These are represented to-day in California, the one by the redwood, the other by the big-tree. They are evidently direct descendants of the Miocene species, though somewhat modified.
But it will be objected that there ought to be some cases of transitional forms showing transmutation—in fact, there ought to be some cases of species now forming under our eyes. There are, we believe, examples of such cases. But intermediate forms are not likely to be maintained long, especially if migrations occur to give rise to severe conflict of forms. In that case the intermediate forms are soon eliminated, and species become distinct. This important point will be discussed more fully in the next chapter.
CHAPTER IX.
PROOFS FROM VARIATION OF ORGANIC FORMS, ARTIFICIAL AND NATURAL.
As already stated, [page 40], the use of the method of experiment in the field of biology is, unfortunately, very limited. Nevertheless, it is already beginning to be used more and more in the department of physiology, and may be used also, to a limited extent, in the department of morphology. It is true that direct scientific experiments, for the express purpose of producing permanent modifications of form, and thus testing the theory of evolution, are of comparatively little value as yet, because the all-important element of time is wanting. The steps of evolution are so slow, and the time necessary to produce any sensible effect is usually so great, that, in comparison, man’s individual lifetime is almost a vanishing quantity. But, from time immemorial, experiments have been unconsciously made by man on domestic animals and food-plants, which bear directly on this subject. All domestic animals and food-plants, and many ornamental flowering plants, have been subjected for ages to a process of artificial selection acting upon natural variation of offspring. As wild species are modified, we believe, indefinitely by divergent variation and natural selection, so domestic species are modifiable certainly largely, perhaps indefinitely, by divergent variation and artificial selection by man. We all know the extraordinary modifications which have thus been gradually brought about in domestic animals, such as dogs, horses, sheep, pigeons, etc.; in food-plants, as cereal grains, garden-vegetables, etc., and in ornamental plants, as roses, dahlias, pinks, etc. We can only give very briefly the principles of the process by which these extreme modifications are produced, referring the reader to works specially devoted to this subject for more complete accounts.
Let it be borne in mind, then (a), that inheritance is not only from the immediate parents, but from the whole line of ancestry. The inheritance from the immediate parents is, doubtless, usually greater than from any other one term of the ancestral series—the effect on the offspring of any previous generation becomes, doubtless, less and less as the distance from the offspring increases—yet the sum of the ancestral inheritance is far greater than the immediate parental. Let it also be borne in mind (b) that true breeding from one form for many generations creates a fund of heredity in that form, and thus tends to produce fixity, rigidity, or permanence in that form.
Fig. 68.
Now, the method of producing artificial breeds, sometimes consciously, sometimes unconsciously, is, briefly, as follows: Suppose it be desired to obtain a variety of an animal, say a dog, having a certain character. We start from a common type, a ([Fig. 68]). If this type were allowed to breed naturally, the slight divergent variation of offspring represented by the radiating lines would neutralize one another by interbreeding, the individual differences would be “pooled” in a common stock, and the species would remain substantially constant. But if among all these slightly divergent varieties we select one, b, which seems in the right direction, and ruthlessly destroy all the others (indicated by crossing them out by the circular line), and breed this variety, b, only, we shall get again a number of divergent varieties. It may be that the larger number of these will be backward, in the direction of the original type a, on account of the ancestral heredity in that direction, but some will again be in the desired direction. Let all the varieties other than the desired one, but especially the backward-going or reverting ones, be again destroyed, and the one kind only selected which seems to be in the right direction, viz., c. As we push the form thus from generation to generation in the desired direction, especially if we attempt to hasten too much the process, the resistance to movement—if I may use the expression—in that direction becomes greater and greater (shown by the decreasing distances between the successive points of divergence, a, b, c, d, etc.), and the tendency to reversion becomes stronger (shown by the greater number and length of the backward-going lines), until finally it is almost impossible to push any farther. We will suppose that x is such a limit. But if, now, we breed true on the point x, destroying the reversions or backward variations for many generations, we will gradually accumulate a fund of ancestral heredity on this point which increases with every added generation, until finally the tendency to reversion becomes small. The variety breeds true without further interference, or with only very general superintendence. Such a permanent variety is called a race. After a race is firmly established for a sufficient length of time, and the tendency to reversion is lost, it may itself become a new point of departure for the formation of new varieties or races, in the same or other directions. Thus, during even the brief history of man, have been formed races of the different domestic animals, and useful and ornamental plants, differing so greatly from each other that, if found in the wild state, they would unhesitatingly be called different species, or even in some cases different genera.