« PreviousContinue »
portance in producing changes in the character of the offspring. Crossing and hybridizing 2 may account for many varieties.3
Of the changes that may be produced by alterations in the medium in which the animal lives, no more striking example could be adduced than one to which allusion has been made in the preceding chapter.
In the summer and autumn of 1871, Schmankiewitsch, a Russian naturalist, noticed that Artemia arietina (a sort of brine-shrimp) found in salt-water pools changed its form according to the greater or less saltness of the water. In summer, when the water was most salt, there was a retardation of growth, which was the more marked the higher the temperature, and the salter the water became. When, near the end of summer, the heavy rains set in, and the temperature decreased, the Artemia became larger, and lost its red and gray color; so that the November and the July broods differed
1 Breeding between two animals of different species, thus producing a mule, or cross.
2 Crossing in plants. The term is applied either to animals or plants, but for convenience it is here used only of the latter.
3 Much may also be allowed for the action of hidden forces at work in or upon the reproductive apparatus, both of animals and of plants; but the action of such forces is at present but little understood.
essentially in size and color. Schmankiewitsch then bred the brine-shrimps artificially. In one vessel he gradually increased the saltness of the water to between four and five times that of ordinary sea-water; 1 in another vessel he reduced the saltness considerably below that
FIG. 2. Brine-shrimp and Young.
1 18° Beaumé.
of sea-water;2 and in both of the series of solutions so prepared he reared several generations. Each new generation lived in a solution of a strength or of a dilution such as the preceding generation could hardly have endured. The new broods of brine-shrimps reared in solutions of varying strength differed greatly from each other, as well as from those in the original pool. Schmankiewitsch also noticed, that in warm
2 30 Beaumé.
weather the females, both in the stronger and in the weaker solutions, reproduced without being fertilized. The females hatched from such unfertilized eggs, themselves, in turn, produced a brood of females only. Males only occurred in water of medium strength. Then, in the spring, after the brine had become suddenly freshened by heavy rains, abnormal males · were produced.1 But the two leading forms which Schmankiewitsch obtained have heretofore been known as two different genera, — Artemia and Branchipus; so that here is a clear case of modifications sufficient to carry an animal from what naturalists rank as one genus into what they call another genus; all taking place by the increase or diminution of salt in the water, more or less aided by variations of temperature.
Yet such changes as this are only immense exaggerations of others, such as are constantly occurring when the conditions of life are changed. The cats of Mombas, on the East African coast, are covered with short, stiff hair, instead of the usual fur; and it is related that common cat taken to Mombas, after only eight weeks' residence there "underwent a com
1 This account is in substance taken from an abstract kindly furnished the authors by Professor A. S. Packard.
plete metamorphosis, having parted with its sandy-colored fur." No doubt, in a generation or two the descendants of any cat taken to this locality would become changed into the variety now characteristic of Mombas. In Paraguay the domestic cats are a fourth smaller than the ordinary European variety, and have a coat of close, short, shiny hair. It is only at Ascension, where the native animals probably often interbreed with those brought from abroad, that the breed at all closely resembles that of other parts of the world. It is a well-known fact, requiring no illustration here, that fur-bearing animals in general differ much in the length and quality of the fur, according to locality and climate; so that the skin, which if taken from an individual living in an arctic or semi-arctic climate would be very valuable, may be worth little or nothing when obtained from the same species in warm-temperate latitudes. It is to the influence of climate, at least in part, that the striking variations in the wildcat genus (to which allusion was made in the preceding chapter) should be referred. Food, too, seems to have, in many cases, a marked effect in producing changes in the external characteristics, in
1 Yet the cats of Paraguay are probably of common descent with those of Europe.
the structure, and in the habits of animals, not only in one generation, but still more decidedly in the course of a series of generations. For example, the domestic cat has intestines a third longer than those of the wildcat; and this fact renders the former animal much more readily able to live, in part or wholly, on vegetable food than the latter would be. Again: there is noticeable, says Mr. Darwin, a tendency, among some civilized races of men, toward losing the usefulness of the wisdom-teeth. In some instances these never appear; while in others they are small, and soon decay. On the other hand, these teeth, in the skulls of certain prehistoric men, were (as shown by skulls and jaws that have been discovered in various parts of Europe) much larger and stronger than in civilized races of the present day. But we know that these prehistoric men lived in the rudest way, partly on the flesh of huge wild animals, which must often have been eaten raw, or nearly so. Their vegetable food must have consisted of nuts, acorns, berries, wild grains, and perhaps roots; and it is easy to see that such a diet must have demanded long canines
1 Animals and Plants, etc., ii., pp. 292, 293. The wildcat is not the ancestor of the domestic breed, but the comparison is at any rate instructive.