THE DEVELOPMENT AND PHYLOGENY OF PLACENTICERAS (CLASSIC REPRINT)

Excerpt from The Development and Phylogeny of PlacenticerasThe student of larval stages must confine himself to the primary sort, if he would correlate them with ancestral genera. The development of the coelenterates, echinoderms, brachiopods, most molluscs, and the lower crustaceans is direct, thus larval stages of these groups may be bearers, to a greater or less degree, of ancestral characters. But Since the free larvae of even these groups are exposed to natural selection, secondary or kenogenetic characters will be introduced, obscuring the resemblance to ancestral forms, also characters that in the adult ancestral form were func tional and fully developed may in the representative larval stage of the descendant be so little differentiated as to be unrecognizable.But how can the morphologist who deals entirely with living Species know whether a character is primary, and repeated by palingenesis in the larval history of the descend ant, or whether it is secondary, and introduced by keno genesis into that history? The answer to this lies wholly within the domain of paleontology, for only by finding a stage of growth represented by an ancestral form can the morphologist know that the characters of that stage are ancestral, and not secondary. Larval stages which may be the bearers of ancestral characters must then be compared with the adults of their predecessors, and the paleontologic record must be invoked as a final resort - the court from which there is no appeal.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in