SPECULATIVE ZOOLOGY. 



373 



quite different when the younger stages are compared, and at first sight 

 no one would suspect that the Sergestes larva (Fig. 7) and the Lucifer 

 larva (Fig. 8) are corresponding stages in the development of two ani- 

 mals as similar to each other as those shown in Figs. 5 and 6. 



Not only do we find animals whose young stages differ more than 

 the adults, but we also meet 

 cases and they are very nu- 

 merous indeed where the or- 

 der of appearance of organs 

 and features of the greatest 

 taxonomic importance differs in 

 the embryos of closely related 

 forms. 



To take a particular instance, 

 it is plain that, since the features 

 which all the two-gilled cepha- 

 lopods have in common, and 

 which are characteristic of the 

 group as a whole, must have 

 been inherited from the com- 

 mon ancestor of the whole 

 group, they ought, unless the 

 embryonic history of the dif- 

 ferent recent species has under- 

 gone secondary modifications, 

 to appear in the same order in 

 the embryos of all the existing 

 forms ; and, if they do not, it 

 is clear that descriptive embry- 

 ology alone can not furnish a 

 key to systematic affinity. 



As a matter of fact, each one 

 of the three species of two-gilled 

 cephalopods with whose embry- 

 ology we are most familiar dif- 

 fers from, both the others in the order in which such significant organs 

 as the arms, the shell, the eyes, the siphon, the gills, and the mouth 

 make their appearance ; and it must be obvious that, unless we have 

 some means of analyzing these three life-histories, and determining 

 which of them gives the true ancestral order, we can not make use of 

 their embryology as a key to phylogeny. One who is not familiar 

 with the whole field of life-science may fairly ask how it is possible 

 to discover the relationships of animals from the study of their embry- 

 ology if it is true that the early stages in the life of closely related 

 species may differ so greatly, and if it is true that the order and man- 

 ner in which structures make their appearance in the embryo are not 



Fig. 8. 



