238 SMITH. [Vol. XVI. 



ing imaginary family trees. To-day a serious naturalist would 

 just as soon think of taking up cudgels in favor of the theory 

 of gravitation as of evolution. Speculation is no longer popu- 

 lar, and now we wish to know, not whether certain organisms 

 developed out of others, but how. This change has largely 

 been brought about by the application of the law of accelera- 

 tion of development to the study of biology. From the studies 

 of Louis Agassiz and his followers we know that, theoretically, 

 each organism in its ontogeny ought to go through stages of 

 growth corresponding to all its ancestors, and that these stages 

 ought to appear in the order of its ancestral forms, A part of 

 this may be verified in biological laboratories, by studying 

 embryonic and larval stages in animals. Even this is difficult, 

 because the habits of larvae are so different at successive periods 

 of growth that, in confinement, it is often impossible to trace 

 them with certainty through the various stages, and we usually 

 have to take different individuals to show the successive devel- 

 opment. And when we attempt to correlate growth stages with 

 ancestral genera the task becomes still more difficult, for then 

 we must leave the living organisms, and are thrown back on 

 paleontology. We often find in a growth stage an association 

 of characters that never occurred in any ancestral form, thus 

 obscuring the parallelism. Then, too, the geologic record is 

 notoriously incomplete, and from the nature of things must 

 always remain so ; thus the paleontologic record is often lack- 

 ing just where we most need it. 



Paleontogeny, 



In order to make a satisfactory comparison of ontogeny and 

 phylogeny, the naturalist must select some group in which 

 living and fossil forms are classified on the same basis ; such 

 groups are the brachiopods and the molluscs. But brachiopods 

 are scarce, hard to obtain by dredging, hard to rear in marine 

 laboratories, and most of the families are long since extinct, so 

 that ontogenic studies of living species have not thrown much 

 light on the history of the race. Beecher, Schuchert, and J. M. 

 Clarke have, however, succeeded in working out the ontogeny 



