Geol.-Vol. I.] SMITH— PLACENTICER AS. 1 83 



preservation of the species are likely to be perpetuated by 

 heredity. Now in the foetal type the most favorable varia- 

 tion consists in abbreviation, thus simplifying the develop- 

 ment. Any characters that are useful in a free state, but 

 not in a foetal state, are liable to be lost. Thus in the foetal 

 type the tendency is towards 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 molluscs^ have the foetal type of de- 

 velopment; 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 sceptical 

 about the repetition of family history in individual develop- 

 ment. Here many stages are omitted, and the rest so 

 obscured and undifferentiated as to be unintelligible; and 

 secondary characters, due to life in the ^^^ 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 indi- 

 cating 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 Larvcs. — If the way of the em- 

 bryologist lies in stony places, that of the student of post- 

 embryonic 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 diffi- 

 culty that there are two distinct types of larvae : (a) primary 



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

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

 relatives. 



