CREATION BY EVOLUTION
conform to the ideal conditions inferred to be the starting point for later advances.
The Mammal-Like Reptiles
We come next to the theromorphs, mammal-like reptiles of Permian and Triassic time, nearly all of which were fierce carnivorous animals. The most primitive of these were lizard-like forms from the Permian rocks of Texas and Russia; but some of them, called pelycosaurs, were specialized side lines. The most noteworthy advance by the earlier members of this group was the development of an opening, or temporal fossa, in the bony mask covering the back part of the skull on each side behind the eyes. The most primitive amphibians and reptiles had this temporal region covered with a continuous shell of bone, but in the mammal-like reptiles the tissue surrounding the jaw muscles (which lay just beneath this bony shell) seems to have acquired the power first of fastening itself to the surface of the bone and then of sinking into the middle of the bony area. Meanwhile the bone itself, while giving way in the middle of the area, strengthened itself around the margins of the area to which the jaw muscle was attached. In this way bony arches grew up around the margins, and an opening appeared toward the middle. Such openings have been developed in different parts of the temporal region in different groups of reptiles; some reptiles had on each side two temporal openings or fossae, one above the other, and other reptiles had none. The mammal-like reptiles had but one, which was surrounded in the later members of the series by the postorbital, parietal, squamosal, and jugal bones. This temporal fossa may be traced through the ascending series of mammal-like reptiles into the early mammals, thence into the tree-shrews, lemurs, monkeys, apes,
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