单项选择题

What is a Species A species of plants or animals, as the term has been generally used by naturalists, comprises all such individuals as are so similar to each other that we may suppose them all to have proceeded from one common parentage, and so dissimilar from all others that they could not have been reproduced from the others, nor the others produced from them. All we know is, that the plants and animals throughout the world exist in species, each one of which stands at present distinct and isolated wholly apart from all the rest, and one cannot be transformed into another by ordinary generation, through changes of soil and climate, or any other causes whatever known to man, within so short a period as six thousand years. The apple, for instance, is one species, and the pear is another. In many respects they are similar to each other, and each may be changed by cultivation and by the operation of other causes a great deal, but by no possibility can one be derived from the other. By different modes of cultivation, by different selections of seeds, by changes in soil, and by other such means, a horticulturist (园艺家) may vary the character of his apples very much. He may produce large apples and small apples, sweet apples and sour apples, apples with a skin red, green, yellow, or brown, but he can never produce a pear. The apple, under all its modifications, will remain an apple still. It is the same with animals. Each one is subject to a great many modifications in respect to its form, its size, its color, and even it faculties, but through all these changes each one remains entirely within its own bounds, as it were. The distinguishing characteristics of the species remain unchanged. Take for instance, any species of the dog. We may, perhaps, by means of differences of treatment, of food, of climate, or of immediate parentage, buy big dogs and little dogs, weak dogs and strong dogs, gentle dogs and fierce dogs, all proceeding from the same original stock. The Distinction of Species Is Very Permanent Although in the comparatively short periods of time, by the experiments and observations which have been made, man can not transform one species into another. Such changes may have been effected in longer periods. Now evidence has shown that the various forms of animal and vegetable life which now exist upon the earth may have proceeded from some common origins, or at least from some moderate number of original types existing in former ages. And, indeed, this may possibly be so. But there seems to be quite satisfactory evidence to prove that the distinction of species is as permanent in respect to the past and the future, at least for very long periods, as it is decisive at the present time. Evidence of Ancient Records In the first place, we have in Egyptian and Assyrian monuments, many drawings and other representations of plants and animals as they existed then, and even seeds, in some cases, found in the wrappings of Egyptian mummies (木乃伊’’), all of which show that these plants and animals, and even the races of men, were specifically the same then as now. There have been no changes whatever that blur (模糊，混淆) the limits and bounds by which the different species are separated from each other at the present day, or confuse the lines of demarcation (界线) in any degree. There is no approach of one type toward another, nor any tendency to such an approach. Between the bird carved upon an Egyptian or Assyrian slab (平板), and its representative at the present day, probably three thousand generations may have intervened, and yet the present living specimen is specifically identical with the delineation of its ancestor. The great comparative anatomist Cuvier examined the mummy of an ibis (朱鹭), from three to four thousand years old, comparing it minutely (细微地) with a living bird of the present day, and found the two specimens in all respects identically the same. There is also a bass-relief from the ruins of Babylon, with a dog represented upon it, which is found by naturalists to be identical with a species of the dog existing in Asia at the present day. Evidence of Fossil Remains But we have still more conclusive evidence than this. It is found in the. fossil remains which exist in the strata of the earth. It also indicates the very great permanence of the characteristics by which different species of plants and animals are distinguished from each other. By means of these, our observations upon the forms of vegetable and animal life which have existed upon our globe may be carried back to an immense antiquity (古老) and extended over so vast a number and variety of species. So, as it has always been supposed, we can get all the means of information on this subject that can be desired. It has been thought to be fully proved by these observations that every species which exists upon the earth remains unchanged so long as it exists. When at length its period has expired, it disappears from the field, while new ones are continually arising to take the place of those that are gone. But no one passes, by gradations, into any other; and the lines of distinction by which each is separated from all the rest remain sharp and well-defined from the beginning to the end. The apple and the pear are of one species.