Page:The Last link.djvu/49

Rh common to mammals, and distinguish them clearly from the other Craniota. This characteristic combination and correlation proves that they have been developed only once in the history of the vertebrate stem, and that they have been transferred by heredity from one common ancestor to all the members of the class of Mammalia.

The next step, as we trace our human phylogeny to its origin, leads us further back into the lower Vertebrata, into that obscure Palæozoic age the immeasurable length of which (much greater than that of the Mesozoic) may, according to one of the newest geological calculations, have comprised about one thousand millions of years.

The first important fact we have to face here is the complete absence of mammalian remains. Instead of these we find in the later Palæozoic period, the Permian, airbreathing reptiles as the earliest representatives of Amniota. They belong to the