Page:Darwinism by Alfred Wallace 1889.djvu/171

VI soil, climate, and atmosphere widely different from those of their native habitat. Thus, many alpine plants only found near perpetual snow thrive well in our gardens at the level of the sea; as do the tritomas from the sultry plains of South Africa, the yuccas from the arid hills of Texas and Mexico, and the fuchsias from the damp and dreary shores of the Straits of Magellan. It has been well said that plants do not live where they like, but where they can; and the same remark will apply to the animal world. Horses and cattle run wild and thrive both in North and South America; rabbits, once confined to the south of Europe, have established themselves in our own country and in Australia; while the domestic fowl, a native of tropical India, thrives well in every part of the temperate zone.

If, then, we admit that when one portion of a species is separated from the rest, there will necessarily be a slight difference in the average characters of the two portions, it does not follow that this difference has much if any effect upon the characteristics that are developed by a long period of isolation. In the first place, the difference itself will necessarily be very slight unless there is an exceptional amount of variability in the species; and in the next place, if the average characters of the species are the expression of its exact adaptation to its whole environment, then, given a precisely similar environment, and the isolated portion will inevitably be brought back to the same average of characters. But, as a matter of fact, it is impossible that the environment of the isolated portion can be exactly like that of the bulk of the species. It cannot be so physically, since no two separated areas can be absolutely alike in climate and soil; and even if these are the same, the geographical features, size, contour, and relation to winds, seas, and rivers, would certainly differ. Biologically, the differences are sure to be considerable. The isolated portion of a species will almost always be in a much smaller area than that occupied by the species as a whole, hence it is at once in a different position as regards its own kind. The proportions of all the other species of animals and plants are also sure to differ in the two areas, and some species will almost always be absent in the smaller which are present in the larger country. These differences will act and react on