THE PALEONTOLOGIC RECORD 295 
PALEONTOLOGY AND ONTOGENY 
By Prorgesssor A. W. GRABAU 
COLUMBIA UNIVERSITY 
eae 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 
