PALEONTOLOGY AND EVOLUTION. The causes of variability among species, the meaning and processes of evolution and natural selection, and the relations between evolution and classification are eonsidered in other articles under those particular titles. The hearing of paleontologic research upon these subjects and some of the results attained deserve brief notice here, and for further information regarding these lines of research the reader is referred to the papers cited in the bibliography at the end of this article. The following lines of research have been dis tinguished: Auxology or Bathmology, the study of growth of organisms; Genesiology, the study of heredity; Ctetology, relating to the origin of acquired characteristics; and Bioplastology, deal ing with the correlation of the ontogeny and phylogeny, or the stages of development with those of evolution.
EMBRYoGENT OF FOSSIL ORGANISMS. Embry onic shells of mollusks. brachiopods, and crusta ceans ale sometimes found as minute in highly fossiliferous shales and limestone. Some adult shells retain at their apices the form of the embryonic shell, and others, like the ammonites, have the shell which hatched from the egg inclosed within the centre of their coiled disks. By examining large numbers of brachio pod pod shells of all sizes. Beecher and Clarke, and later Schuchert, were able to arrange the in dividuals of certain species in series according to size, and to show that they all were derived from an embryonic stage, called the protegulum, of very simple form, resembling the Cambrian genus Paterina. They have shown that members of all the families of brachiopods began their existence as paterina-like shells, and that the distinctive adult shapes have been attained through modifications in the mode of growth of the shell during the stages subsequent to the proteguluni stage. Beecher has shown that mem bers of the principal families of trilobites began their existence as embryonic forms, called the `protaspis,' which is comparable with the pro touauplius stage of the more primitive living Among the corals, several fossil genera. as Favosites, Syringopora. etc. (fide
Beecher and Girty), pass through an embryonic stage that resembles another fossil coral Anlopora. The nautiloid and ammonoid cephalopods present the most favorable conditions for the preserva tion of the embryonic stages of growth, because their shell; hold within their centres all the successive stages through w hid they have 'Kissed in their ontogenetie development. By breaking open such a shell the devdopment al stages can be studied from the earliest •protoeonel ' hatched from the egg awl found at the centre 4 f the coil. to the senile or old age stage represented by the last chamber in which the animal lived. See CEPHALoPODA.
AGAssIZ's I...kw OF ly termed by Haeckel the law of palingene-is, according, to which the of development or ontogeny of the individual can be err lated wit!' the stages, of evolution or illiylogeny of the race to which the particular indivi lual belongs. has received abundant confirmative evidt nee Irian paleontology. and the literature on this and allied branches of research is quite formidable. c-pe chilly in its technicality of expression. The b lowing scheme of terms adapted from Hyatt 11.1i been devised to distinguish the corresponding stages: Fine illustration' of such correlation- between ontogenetic and phylogenetic stages have been fur nished by Ilyatrs study of the Arieti Le. a group of ammonites; by Beecher's demonstration of the phylogeny of the Terebratellidie. a family of brachiopods; and by Beecher's studies on the larval forms of trilobites. This palingenctie law is of much value to the stratigraphic paleon tologist, for it enables him to correlate geological formations of which the faunas consi-t of wholly unknown species. It also enables him to postu late the existence in earlier formations of un known genera which when found will prove to be eounterparts of larval or adolescent stages of speeies already under observation.