tilian or amphibian skull, but there were twice or thrice as many bones in the older forms as there are in recent ones. And there has been in general an increase in bodily size in every phylum. The largest animals have always lived at or near the end of their race, and a race of small animals has never been evolved from a race of large animals. Furthermore, horns, spines, protuberances, and excrescences occur only in the later history of any race, never at its beginning.
The two chief factors of evolution have been environment and heredity. There is more or less impulse due to heredity, a sort of vis a tergo that seems to influence evolution along parallel lines in related forms, though we are never sure how much is due to it and how much to similar environmental influences.
The chief problem, then, in any classification is the relative importance of structural characters in the absence of the actual connecting links insensibly uniting different forms, that is, the determination of the more conservative hereditary characters of the skeleton, those which have been influenced less by environmental conditions. Some parts of the skeleton are very variable even in nearly related forms. The number of vertebrae in the spinal column, we have seen, may vary extraordinarily within an order. Chameleon lizards have only about sixty vertebrae; other lizards may have a hundred and ninety-four, while snakes of the same order may have as many as four hundred and fifty. The number is seldom of more than generic value, and sometimes perhaps not more than specific. It would be absurd, for instance, to unite in the same group a lizard and a turtle because each happens to have eight cervical vertebrae.
And the teeth of reptiles, unlike those of mammals, have little value as criteria of relationships, so adaptable are they in shape and number to food habits, though their location may be more conservative. The pectoral and pelvic girdles have been influenced less by environmental conditions; the structure of the feet still less in adaptation to life conditions. More conservative is the arrangement and mode of articulation of the ribs. Most conservative of all has been the structure of the cranial region of the skull, that surrounding the brain, and in consequence it furnishes the most reliable characters for the discrimination of the larger groups, the subclasses or superorders.
