"The change from plantigrade to digitigrade, with increasing elevation of the heel, when taken in connection with increasing size of the brain, and therefore presumably with increasing brainpower, shows a gradual improvement of structure adapted for speed and activity, and a pari-passu increase of nervous and muscular energy necessary to work the improved structure." 1 The foregoing argument is just like that regarding the words âme and anima; Eohippus so closely resembles Orohippus that they must be the same; Orohippus must be the same as Mesohippus; Mesohippus must be the same as Protohippus; Protohippus is the same as Pliohippus; Pliohippus is the same as Equus; therefore the modern horse is the same as the Eohippus. The force of this argument will depend upon the strength of the impressions of resemblance made upon various minds. Professor Huxley regarded it as demonstrative. 1 Pages 540-543. CHAPTER VIII. FACTS OF COEXISTENCE. EVERY observer very quickly perceives that the various objects in the world may be divided into groups of permanent coexistences. Here is a mass of matter with specific gravity 19.34, a yellow color, malleable, ductile, etc., and there is another mass of matter in which the same phenomena coexist, and there is another. We call all these masses gold; and we say that gold is a kind of matter. Malleability, ductility, etc., are commonly called the properties of gold. But in truth we know absolutely nothing about gold except these properties. The weight does not possess the ductility, nor does the color possess the malleability; but the coexistence of all these phenomena is gold. No approach has been made by science to any reason why certain phenomena permanently coexist; as, for instance, why the metal whose specific gravity is 19.34 should be yellow, and the metal whose specific gravity is 10.5 should be white. It is easy to say that all the properties probably depend upon some common fact of causation; but in the present state of science such a remark has no meaning. A very large part of the work of science is in ascertaining the various natural kinds of objects. Mr. Mill magnifies the notion of cause and calls it "the root of the whole theory of induction." But it is plain that the notion of coexistence is an equally important root. We cannot reason that such and such things must coexist; we can only discover that they do. This work has nothing to do with causation. It has nothing to do with the unknown. It does not proceed by inference. It is the orderly arrangement of what we know. One vast attempt of Induction is to classify the objects in nature, that is, to discover and define all natural kinds. In this attempt it is soon perceived that there are groups within groups. Vegetables, for example, are a natural kind; but the vegetable kingdom may be subdivided into more limited kinds, and these kinds may be again subdivided. A distinction is made between Natural and Artificial kinds. We may, for temporary convenience, divide objects according to some one property, as yellowness. And then gold and oranges and salmon will be of the same kind. Such a group is called an Artificial Kind. But Natural Kinds are so called because the objects which compose them resemble each other in a multitude of characteristics and appear, in fact, grouped together by nature. The great botanist Linnæus systematized plants according to the numbers of stamens and pistils, neglecting other features. This was a convenient, but highly artificial, arrangement; since it brought into the same order plants on the whole utterly diverse. Modern botany takes into consideration a multitude of particulars in stem, leaf, flower, and fruit; and so reaches a natural system. No classification is natural which depends in the least degree upon the caprice of the investigator; it must force itself upon all observers as existing in nature. That there is a kind of objects which we may call plants and another kind of objects which we may call animals is generally admitted. But when we come to subdivide the animal and vegetable kingdoms, differences of opinion arise. It is obvious that certain individuals greatly resemble one another; they constitute natural groups, which may be called species. Certain species resemble one another; they may be associated in larger groups and called genera. So the genera may be grouped into orders, and the orders into classes. Philosophers have discussed the question whether there is a point where natural subdivision ends. If there is such a point, then one of the smallest possible natural groups would be called an infima species. If, on the other hand, there be a group which cannot naturally be included in a larger, such a group would be called a summum genus. The most interesting question in modern natural science is, whether the various natural groups of animals and plants species, genera, orders, etc. — are naturally separated by distinct lines. The discussion has taken the form of an inquiry into the true nature of species. The main points in it can be conveniently presented in the words of Professor Asa Gray: "The ordinary and generally received view assumes the independent, specific creation of each kind of plant and animal in a primitive stock, which reproduces its like from generation to generation, and so continues the species. Taking the idea of species from this perennial succession of essentially similar individuals, the chain is logically traceable back to a local origin in a single stock, a single pair, or a single individual, from which all the individuals composing the species have proceeded by natural generation. Although the similarity of progeny to parent is fundamental in the conception of species, yet the likeness is by no means absolute; all species vary more or less, and some vary remarkably — partly from the influence of altered circumstances, and partly (and more really) from unknown constitutional causes which altered conditions favor rather than originate. But these variations are supposed to be mere oscillations from a normal state, and in Nature to be limited if not transitory; so that the primordial differences between species and species at their beginning have not been effaced, nor largely obscured, by blending through variation. Consequently, whenever two reputed species are found to blend in Nature through a series of intermediate forms, community of origin is inferred, and all the forms, however diverse, are held to belong to one species. Moreover, since bisexuality is the rule in Nature (which is practically carried out, in the long run, far more generally than has been suspected), and the heritable qualities of two distinct individuals are mingled in the offspring, it is supposed that the general sterility of hybrid progeny interposes an effectual barrier against the blending of the original species by crossing, "From this generally accepted view the well-known theory of Agassiz, and the recent one of Darwin, diverge in exactly opposite directions. "That of Agassiz differs fundamentally from the ordinary view only in this, that it discards the idea of a common descent as the real bond of union among the individuals of a species, and also the idea of a local origin- supposing, instead, that each species originated simultaneously, generally speaking, over the whole geographical area it now occupies, or has occupied, and in perhaps as many individuals as it numbered at any subsequent period. "Mr. Darwin, on the other hand, holds the orthodox view of the descent of all the individuals of a species not only from a local birthplace, but from a single ancestor or pair; and that each species has extended and established itself, through natural agencies, wherever it could; so that the actual geographical distribution of any species is by no means a primordial arrangement, but a natural result. He goes farther, and this volume [The Origin |