Palaeozoic period, could we find them, might contain other orders of vertebrata.  But no such reply can be made to the argument that whereas the marine vertebrata of the Palaeozoic period consisted entirely of cartilaginous fishes, the marine vertebrata of later periods include numerous genera of osseous fishes; and that, therefore, the later marine vertebrate faunas are more heterogeneous than the oldest known one.  Nor, again, can any such reply be made to the fact that there are far more numerous orders and genera of mammalian remains in the tertiary formations than in the secondary formations.  Did we wish merely to make out the best case, we might dwell upon the opinion of Dr. Carpenter, who says that “the general facts of Palaeontology appear to sanction the belief, that the same plan may be traced out in what may be called the general life of the globe, as in the individual life of every one of the forms of organised being which now people it.”  Or we might quote, as decisive, the judgment of Professor Owen, who holds that the earlier examples of each group of creatures severally departed less widely from archetypal generality than the later ones—­were severally less unlike the fundamental form common to the group as a whole; that is to say—­constituted a less heterogeneous group of creatures; and who further upholds the doctrine of a biological progression.  But in deference to an authority for whom we have the highest respect, who considers that the evidence at present obtained does not justify a verdict either way, we are content to leave the question open.

Whether an advance from the homogeneous to the heterogeneous is or is not displayed in the biological history of the globe, it is clearly enough displayed in the progress of the latest and most heterogeneous creature—­Man.  It is alike true that, during the period in which the Earth has been peopled, the human organism has grown more heterogeneous among the civilised divisions of the species; and that the species, as a whole, has been growing more heterogeneous in virtue of the multiplication of races and the differentiation of these races from each other.

In proof of the first of these positions, we may cite the fact that, in the relative development of the limbs, the civilised man departs more widely from the general type of the placental mammalia than do the lower human races.  While often possessing well-developed body and arms, the Papuan has extremely small legs:  thus reminding us of the quadrumana, in which there is no great contrast in size between the hind and fore limbs.  But in the European, the greater length and massiveness of the legs has become very marked—­the fore and hind limbs are relatively more heterogeneous.  Again, the greater ratio which the cranial bones bear to the facial bones illustrates the same truth.  Among the vertebrata in general, progress is marked by an increasing heterogeneity in the vertebral column, and more especially in the vertebrae constituting the