Behind this again comes the quaternary or pleistocene period, so called from its fauna, which, although containing extinct species, shows along with them many existing forms, some of which have migrated and some remain. This also may be called the glacial period, for although the commencement, termination, and different phases of the two great glaciations and intermediate inter-glacial periods cannot be exactly defined, and hard-and-fast lines drawn between the later pliocene at one end and the post-glacial at the other, there is no doubt that in a general way the quaternary and glacial periods coincide, and that the changes of climate were to a considerable extent the cause of the changes of flora and fauna. Behind the quaternary comes in the tertiary, with its three great divisions of Pliocene, Miocene, and Eocene, each containing numerous subdivisions, and all showing a progressive advance in forms of life, from older and more generalized types towards newer and more specialized ones, and a constant approach towards genera and species now existing. Behind the tertiary comes the secondary period, into which it is unnecessary to enter for the present purpose, for all is different, and even mammalian life is only known to be present in a few forms of small and feeble marsupials. Nor is it necessary to enter on any detailed consideration of the Eocene or earlier tertiary, for the types of mammalian life are so different from those of later periods, that it cannot be supposed that any animal so highly organized as man had then come into existence. The utmost we can suppose is that, as in the case of the horse, some ancestral form from which the quadrumana and man may possibly have been developed may be found. But up to the present time nothing has been found in the Eocene more nearly approaching such a missing link than an ancient form of lemur; and it is not until we get into the Miocene that any evidence presents itself that man, or some near ancestor of man, may possibly have existed.[8]