54 THE PALEONTOLOGIC RECORD 



PALEONTOLOGY AND ONTOGENY 



BY PKOFESSSOR A. W. GRABAU 



COLUMBIA UNIVERSITY 



ONTOGENY, 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 zool- 

 ogists 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 develop- 

 ment. 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 paleontolo- 

 gist 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 modifica- 

 tion, 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 



