Evolution and Adaptation

by Thomas Hunt Morgan

In regard to plants, the influence of the environment has long been known to produce an effect on the form, color, etc., of the individuals. “The common dandelion (Taraxacum densleonis) has in dry soil leaves which are much more irregular and incised, while they are hardly dentate in marshy stations, where it is called Taraxacum palustre.

“Individuals growing near the seashore differ markedly from those growing far inland. Similarly, species such as some Ranunculi, which can live under water as well as in air, exhibit marked differences when considered in their different stations, as is well known to all. These differences may be important enough to induce botanists to believe in the existence of two different species when there is only one.”

An interesting case is that of Daphnia rectirostris, a small crustacean living sometimes in fresh water, at other times in water containing salt and also in salt lakes. There are two forms, corresponding to the conditions under which they live, and it is said that the differences are of a kind that suffice to separate species from each other. In another crustacean, Branchipus ferox, the form differs in a number of points, according to whether it lives in salt or in fresh water. Schmankewitsch says that, had he not found all transitional forms, and observed the transformation in cultures, he would have regarded the two forms as separate species. The oft-quoted case of Artemia furnishes a very striking example of the influence of the environment. Artemia salina lives in water whose concentration varies between 5 and 12 degrees of saltness. When the amount of salt is increased to 12 degrees, the animal shows certain characteristics like those of Artemia milhausenii, which may live in water having 24 to 25 degrees of saltness. The form A. salina may be further completely changed into that of A. milhausenii by increasing the amount of salt to the latter amount.

Among domesticated animals and plants—a few instances of which have been already referred to—we find a large number of cases in which a change in the environment produces definite changes in the organism. Darwin has made a most valuable collection of facts of this kind in his “Animals and Plants under Domestication.” He believes that domesticated forms are much more variable than wild ones, and that this is due, in part, to their being protected from competition, and to their having been removed from their natural conditions and even from their native country. “In conformity with this, all our domesticated productions without exception vary far more than natural species. The hive-bee, which feeds itself, and follows in most respects its natural habits of life, is the least variable of all domesticated animals.... Hardly a single plant can be named, which has long been cultivated and propagated by seed, that is not highly variable.” “Bud-variation ... shows us that variability may be quite independent of seminal reproduction, and likewise of reversion to long-lost ancestral characters. No one will maintain that the sudden appearance of a moss-rose on a Provence rose is a return to a former state, ... nor can the appearance of nectarines on peach trees be accounted for on the principle of reversion.” It is said that bud-variations are also much more frequent on cultivated than on wild plants.

Darwin adds: “These general considerations alone render it probable that variability of every kind is directly or indirectly caused by changed conditions of life. Or to put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life, there would be no variability.”

In some cases it has been observed that, in passing from one part of a continent to another, many or all of the forms of the same group and even of different groups change in the same way. Allen’s account of the variations in North American birds and mammals furnishes a number of striking examples of this kind of change. He finds that, as a rule, the birds and mammals of North America increase in size from the south northward. This is true, not only for the individuals of the same species, but generally the largest species of each genus are in the north. There are some exceptions, however, in which the increase in size is in the opposite direction. The explanation of this is that the largest individuals are almost invariably found in the region where the group to which the species belongs receives its greatest numerical development. This Allen interprets as the hypothetical “centre of distribution of the species,” which is in most cases doubtless also its original centre of dispersal. If the species has arisen in the north, then the northern forms are the largest; but if it arose in the south, the reverse is the case. Thus, most of the species of North America that live north of Mexico are supposed to have had a northern origin, as shown by the circumpolar distribution of some of them and by the relationship of others to Old World species; and in these the largest individuals of the species of a genus are northern. Conversely, in the exceptional cases of increase in size toward the south, it can be shown that the forms have probably had a southern origin.

The Canidæ (wolves and foxes) have their largest representatives, the world over, in the north. “In North America the family is represented by six species, the smallest of which (speaking generally) are southern and the largest northern.” The three species that have the widest ranges (the gray wolf, the common fox, and the gray fox) show the most marked differences in size. The skull, for instance, of “the common wolf is fully one-fifth larger in the northern parts of British America and Alaska than it is in northern Mexico, where it finds the southern limit of its habitat. Between the largest northern skull and the largest southern skull there is a difference of about thirty-five per cent of the mean size. Specimens from the intermediate region show a gradual intergradation between the extremes, although many of the examples from the upper Missouri country are nearly as large as those from the extreme north.” The common fox is about one-tenth larger, on the average, in Alaska than it is in New England. The gray fox, whose habitat extends from Pennsylvania southward to Yucatan, has an average length of skull of about five inches in the north, and less than four in Central America—about ten per cent difference.

The Felidæ, or cats, “reach their greatest development as respects both the number and the size of the species in the intertropical regions. This family has sent a single typical representative, the panther (Felis concolor), north of Mexico, and this ranges only to about the northern boundary of the United States. The other North American representatives of the family are the lynxes, which in some of their varieties range from Alaska to Mexico.” Although they vary greatly in different localities in color and in length and texture of pelage, they do not vary as to the size of their skulls. On the other hand the panther (and the ocelots) greatly increases in size southward, “or toward the metropolis of the family.”

Other carnivora that increase in size northward are the badger, the marten, the fisher, the wolverine, and the ermine, which are all northern types.

Deer are also larger in the north; in the Virginia deer the annually deciduous antlers of immense size reach their greatest development in the north. The northern race of flying squirrels is one-half larger than the southern, “yet the two extremes are found to pass so gradually one into the other, that it is hardly possible to define even a southern and a northern geographical race.” The species ranges from the arctic regions to Central America.