608 



THE AMERICAN NATURALIST 



[Vol. XLI 



theory in an evil light, and awakened in the minds of many serious 

 investigators doubts as to the validity of the deductions based upon 

 this doctrine. When, however, the entire life history of the indi- 

 vidual is considered, instead of only the embryonic period, and when 

 the successive stages of epembryonic development are compared 

 with the adult characters of related types in immediately preceding 

 geologic periods, it will be found that the fundamental principle of 

 recapitulation is sound, and that the individuals do repeat in their 

 own epembr}'onic development the characters of their immediate 

 ancestors. 



One of the great mistakes made by the majority of systematists 

 is the disregard of the immature stages of development; i. e., the 

 stages between the embryonic and adult. This is notably the case 

 among writers on recent mollusks, who either ignore the early 

 stages entirely in their specific description or give them the briefest 

 notice. And yet it is in these early stages that we find the key to 

 the affinities of a given species with others of its kind, in the present 

 and in past faunas, more often and more surely than in the adult 

 characters. To classify by adult characters only is to neglect the 

 nearest and most obvious method for the ascertainment of the line 

 of descent of the species in question; and, further, it is to leave out 

 of consideration the inevitable similarity produced in the aspect of 

 adult types of different origins, by a loss of the characters distinc- 

 tive of their respective ancestors and of their younger stages. The 

 classification into one family of all l)ald headed men of the same 

 age would not be more illogical than some of the classifications of 

 phylogerontic mollusks in vogue today, — classifications based 

 wholly on adult characters. Agassiz long ago called attention to 

 the need of considering the stages between the embry^o and adult, 

 as the following extracts will show. 



" Embryologists have generally considered their work as com- 

 pleted when they have traced the new being to a point at wliich it 

 resembles somewhat any of the nu'inlxTs of the niitiiral o-roup to 

 which it belongs. The process liy which the u'nuhial conipU-tion 

 of the whole frame is attained has \)vv\\ as>nin(>(l to he of Httle 

 interest, hardly deserving the carcfiil -.cnitiny of i\\v (Mnbryologist; 

 while the zoologist has also ovcrlooki.-d, or icuanled as of little 

 importance, the differences which still <listinguisli the young from 



