Samuel Wendell Williston

“ Nevertheless, in tracing the genealogies of organisms, that is, in classifying them, we are aided by general laws which have obtained recognition among students of extinct animals. First of all, by the law that evolution is irreversible, that organs or functions once lost can never be regained by descendants ”

“ In all probability the earth since remote ages has always been as densely populated with living organisms as it is at the present time, and rapidly or slowly in different kinds of organisms evolution and extinction have replaced the faunas and floras many times. There are to-day living upon the earth about twenty thousand species of air-breathing vertebrate animals, and doubtless there has been no time since the first general invasion of land by air-breathers that the number has been less; it may have been greater, since man has exerted a powerful influence upon them. ”

“ By the general law that there has been a continuous loss of parts; we can trace, for instance, probably every bone of the human skull back to the primitive reptilian 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. ”

“ One who has studied attentively the skeleton of reptiles cannot fail to be impressed with the fact that similar or even apparently identical structures have arisen in different orders. Procoelous vertebrae, for instance, occur in crocodiles, pterodactyls, lizards, and frogs when it seems impossible that all should have been evolved from the same common ancestor with procoelous vertebrae. Snakes, some lizards, and certain Stegocephalia have a peculiar mode of articulation of the vertebrae, called zygosphenal, but their evolution from a common ancestor is impossible. ”

“ 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. ”

“ As the only air-breathers of paleozoic times were amphibians and reptiles, there must have been, during the time that they reigned supreme,—from the Mississippian to the Jurassic, millions of years,—scores of thousands of their kinds; we know but a few hundreds. Had we records of all that have lived, the major problems would be much easier, the minor ones greatly increased. ”