414 JAMES PERRIN SMITH 



Secondary elements will be introduced in either type of devel- 

 opment, and those variations that are favorable to the preserva- 

 tion of the species are likely to be perpetuated by heredity. 

 Now in the foetal type the most favorable variation consists in 

 abbreviation, thus simplifying the development. Any charac- 

 ters that are useful in a free state, but not in a fcetal state, are 

 liable to be lost. Thus in the fcetal type the tendency is toward 

 loss of the record through omission of stages or obscuring them, 

 for many organs that would be highly developed in mature forms, 

 or in free larvae, will be either suppressed or undifferentiated. 



The vertebrates, most of the higher crustaceans, most land and 

 fresh-water mollusks * have the foetal type of development ; and 

 these embrace by far the larger part of animals whose ontogeny 

 has been studied. It is not to be wondered at, then, that 

 morphologists who deal exclusively with embryonic stages of 

 these groups should be skeptical about the repetition of family 

 history in individual development. Here many stages are omit- 

 ted, and the rest so obscured and undifferentiated as to be unin- 

 telligible; and secondary characters, due to life in the egg or in 

 the parent, are introduced, effacing what little meaning was left. 

 Then, too, embryologists are often content to trace the animal 

 but a little way toward perfection of development; they study 

 the embryo until the cells begin to divide into groups indicating 

 a beginning of organs, and call this studying ontogeny, when 

 they have stopped before it could be told whether the animal 

 was going to develop into fish, flesh, or fowl. To this sort of 

 study is due the idea of "falsification of the record," a crime of 

 which nature has not yet been guilty, although she at times may 

 not, perhaps, have told the whole truth. 



Primary and secondary larva. — If the way of the embryologist 

 lies in stony places, that of the student of postembryonic stages 

 is not much smoother ; formidable obstacles meet him on every 

 side, reducing his small stock of faith. At the very outset he is 

 confronted by the difficulty that there are two distinct types of 



1 Dreissensia, a fresh-water pelecypod, which in very recent geologic time has 

 immigrated from salt water, still goes through its larval development, like its marine 

 relatives. 



