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the equatorial and polar regions, or between the sea and the land, &c.—all such currents must be due to that source from which the varying quantities of heat proceed.1
We have thus illustrated the law of Evolution in the departments of astronomy and geology, because the facts of these sciences have a close bearing upon our special subject. It would have been still more easy to trace the operation of the law in the development of languages, or in the genesis of an individual animal from parents of the same kind.
1 See H. Spencer's First Principles, p. 207. For most of the facts of this section the writer is indebted to Sir C. Lyell's Principles of Geology.
THE LIVING THINGS OF THE PRESENT AND THE PAST.
§ 1. Some Facts of Natural History.
THE living creatures on the face of the earth are numbered by millions of millions-animal and vegetable; inhabitants of the land, the air, the waters, of visible size or microscopic-and no one is exactly like another. When we have to do with numerous objects, we find it convenient to classify them according to their resemblances and differences; but if their similarities and distinctions are numerous, a variety of arrangements becomes possible; and the question arises, Which arrangement is the best or most natural? In a library, a child may place books in the order of their sizes, or according to the styles of their bindings; and perhaps even in the alphabetical succession of the authors' names, so that any particular book may easily be found. The librarian, passing over such superficial resemblances, distributes them according to their subjects, putting together in one great division all works on history; in another, all biographical works; in another, all works that treat of science; in another, voyages and travels; and so on. Each of his great groups he separates into sub-groups; as when, having divided his scientific treatises into abstract and concrete, putting in the one logic and mathematics, and in the other physics, astronomy, geology, chemistry, physiology, &c., he goes
on to sub-divide his books on physics into those which treat of mechanical motion; those which treat of heat; those which treat of light, of electricity, of magnetism. This arrangement, being according to combinations of attributes, is superior to the child's arrangement, which is in accordance with a single attribute; but it requires more intelligence to make it; and since the attributes, though fundamental, are not conspicuous, it requires analysis, and does not suggest itself till analysis has made some progress.1
The first step taken by the naturalist in the classification of living things, is to divide plants from animals. From the microscopic, formless dab of jelly, which constitutes the Amœba, up to the marvellously complex structure which we name Man, there is, underlying all diversities, a community of traits on which we found the group Animal. In classifying these diversities, we establish groups and sub-groups as indications of the degrees of unlikeness. When the animals differ but slightly, we group them as varieties; when they differ more, as species; when they are still more different, as genera; and so on through families, orders, classes, subkingdoms.
The examination necessary for this classification makes us acquainted with several important facts. In the first place, we find that animals are not independently organized. It is conceivable that every animal should have been constructed on a plan of its own, having no resemblance whatever to the plan of any other animal; and there is nothing in the nature of the case to lead us to suspect a community of organization between animals
1 See H. Spencer's Principles of Biology. Chapter on Classification.
ANIMAL FORMS CONNECTED.
so different in habit and appearance as a porpoise and a gazelle, or a butterfly and a lobster. But, as a matter of fact, the different members of the animal kingdom, from the highest to the lowest, are marvellously connected. Every animal has a something in common with all its fellows, much with many of them, more with a few, and usually so much with several that it differs but little from them. The names by which we designate groups and sub-groups have, therefore, a fixed meaning; they point to morphological and structural resemblances in the things classified, and are a statement of the gradations of likeness observable. But this similarity of structure, often masked under outward difference of form, suggests many questions. Why should the dog, the eagle, the snake, the frog, and the salmon each have an internal skeleton composed of bone or cartilage, and forming an envelope to the nervous centres? Why should insects so different in outward shape as the dragon-fly, the lady-bird, the butterfly, and the flea, have this in common with one another, and with crabs and lobsters, that there are primarily twenty segments to the body? It does not seem necessary, in the sense that no other number would have made a possible organism, and it cannot be by chance.
In the second place, we find that it is difficult to determine what is a Species. It must not be supposed that species, genera, orders, and classes are assemblages of definite values; that every genus is equivalent to every other genus in respect of its degree of distinctness; and that orders are separated by lines of demarcation that are as broad in one place as another. Small differences are observed between animals and their off1 Huxley's Classification of Animals.
spring; greater differences are observed between varieties known to be sprung from a common stock; but the differences between what have been termed species are sometimes hardly greater than those between varieties owning a common origin. What is a species? Species has been defined as "a succession of individuals capable of reproducing themselves;" but the offspring are not exactly like the parents, and it is not always easy to say what the parentage has been. Species, says Mr Lewes, is a subjective creation, having no objective existence; it is an idea, not a thing; a systematic artifice, not a living entity.1 In determining whether two or more allied forms ought to be ranked as species or varieties, naturalists are practically guided by the following considerations-namely, the amount of difference between them, the relation of such differences to few or many points of structure, their degree of physiological importance, but more especially their constancy or inconstancy. Still, the work of classification is not hereby freed from all difficulty, and disputes continually arise on the questions, Whether such and such organisms are specifically or generically distinct; and whether this or that peculiarity is, or is not, of ordinal importance? The little marine creatures, called Foraminifera, were divided by D'Orbigny, and other authors, into a number of clearly defined families, genera, and species; but the researches of Dr Carpenter and his colleagues have shown that the range of variation in the group is so great as to include not merely those differential characters which have been usually accounted specific, but also those upon which the greater part of the genera have been founded, and even, in some instances, those of 1 Fortnightly Review, April 1868.