are without them. The same arguments apply to many of the mechanical contrivances by means of which animals have accommodated themselves to their physical surroundings. Locomotion by land, air, or water was primarily effected in each case by the adjustment of small increments of variation extending over long periods of time; so that the teleological argument for design in nature resolves itself into the slow process of imperceptible changes in the environment, the effect of which may ultimately produce a variation of species.
The Line of Man's Ascent.
The result of the above brief survey of some of the laws and phenomena of organic life, past and present, is to show that while the vast majority of living things have not risen high up in the scale of evolution—some remaining for ages on a comparatively low plane, or even degenerating, and others becoming for ever extinct—there are a few which have made extraordinary advances, both as regards differentiation of parts and specialisation in function. That the chronological sequence of animal development in bygone ages was from the less to the more highly specialised, will be apparent by a glance at a table of the stratified rocks containing typical fossil remains. Starting with the Palæozoic period, and passing in succession through the more recent strata of the earth's crust, we find the following sequence in the appearance of the Vertebrata—Fishes, Reptiles, Birds, Mammals, and last of all, Hominidæ. That is to say, that during the Cambrian period only invertebrate animals flourished; in the Devonian, the previous animal life was increased by the addition of fishes; in the Lower Mezozoic, reptiles for the first time appeared on the scene; but not till the Cretaceous period have we evidence of the existence of birds and mammals. Finally, Man, the most highly differentiated of all animals, made his debut among the inhabitants of the globe, an event which probably took place towards the close of the Tertiaries.
Embryology.
The above chronological sequence in the development of animal life has a remarkable parallelism in the phenomenon of
