For example, if, as assumed in the theory, a group of individuals (species) breaks up into two groups, each of these may be supposed to inherit a large number of common characteristics from their ancestors. These characters are, of course, the resemblances, and from them we conclude that the species are related and, therefore, we put them into the same genus. The differences, as has been said, between the species must be explained in some other way; but the principle of classification with which we are here concerned is based simply on the resemblances, and takes no account of the differences between species. In this argument it has been tacitly assumed that the transformation of one species into another, or into more than one, takes place by adding one or more new characters to those already present, or by changing over a few characters without altering others. But when we come to examine any two species whatsoever, we find that they differ, not only in one or in a few characters, but in a large number of points; perhaps in every single character. It is true that sometimes the differences are so small that it is difficult to distinguish between two forms, but even in such cases the differences, although small, may be as numerous as when they are more conspicuous. If, then, this is what we really find when we carefully examine species of animals or of plants, what is meant when we claim that our classification is based on the characters common to all of the forms that have descended from the same ancestor? We shall find, if we press this point that, in one sense, there is no absolute basis of this sort for our classification, and that we have an unreal system. If this is admitted, does our boasted system of classification, based as it is on the principle of descent, give us anything fundamentally different from an artificial classification? A few illustrations may make clearer the discussion that follows. If, for example, we take a definition of the group of vertebrates we read: “The group of craniate vertebrates includes those animals known as Fishes, Amphibians, Reptiles, Birds, and Mammals; or in other words, Vertebrates with a skull, a highly complex brain, a heart of three or four chambers, and red blood corpuscles.” If we attempt to analyze this definition, we find it stated that the skull is a characteristic of all vertebrates, but if we ask what this thing is that is called skull, we find not only that it is something different in different groups, being cartilaginous in sharks, and composed of bones in mammals, but that it is not even identical in any two species of vertebrates. If we try to define it as a case of harder material around the brain, then it is not something peculiar to the vertebrates, since the brain of the squid is also encased in a cartilaginous skull. What has been said of the skull may be said in substance of the brain, of the heart, and even of the red blood corpuscles.