Page:Popular Science Monthly Volume 77.djvu/301

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NTOGENY, or the life history of the individual, is commonly interpreted by zoologists as its embryology, the later stages of development, from infancy to old age, being deemed of little or no importance. This was the case fifty years ago; this is largely the case to-day. From the days when Agassiz first called the attention of zoologists to their one-sided attack of the problem of ontogeny, and urged them to pay attention to the important post-embryonic stages, down to our own time, students of recent animals have for the most part been content to follow the beaten path. They have left to the paleozoologist the study of the later stages in the life history of the individual, and the latter's endeavors in this direction have developed the science of zoontogeny as to-day understood. There was, perhaps, a natural cause for this separation, in the fact that the student of soft tissues finds few changes which he deems worthy of attention, between the embryo and the adult; whereas the student of hard structures generally sees an abundance of such changes. This is especially true of invertebrates, more particularly of such as build external hard structures in which successive additions are marked by the lines of growth. Vertebrates, and invertebrates without permanent hard parts, such as the Crustacea, require series of individuals showing the successive steps in development. But mollusks, brachiopods and corals show, by their incremental lines, the steps in the life history during the post-embryonic period, so that one perfect individual suffices to present these later stages in development.

It is not infrequently urged that the hard parts of invertebrates, especially the shells of mollusks, are not reliable indices of ontogenetic development, since they represent only the integument, which is subject to ready modification under the influence of the environment. Such an argument is based on a total ignorance of the relation of the shell or other hard structure to the soft parts of the animal. The paleontologist is convinced that the hard parts of animals are the best indices of its development, since they record in a permanent form all the minute modifications which are not even recognizable in the soft parts. More than this, I believe that shells, those of mollusks at any rate, furnish us with a record of changes wholly independent of the environment, and referable entirely to an inherited impulse towards progressive modification, along definitely determinable lines. I am well aware that I am not expressing the opinion of all paleontologists in this statement, and that this view, moreover, is strongly opposed by some of our ablest European conchologists. But here again I contend that this difference