THEORY OF SEX-DIMORPHISM 89 



And if it be said that the cases we would 

 adduce are not fair samples of sex-characters, 

 we would reply that it is very difficult to 

 draw a line round " secondary sex-characters," 

 separating them from other sex-differences. 

 This is especially difficult among invertebrate 

 animals where we have little knowledge of 

 glands of internal secretion connected with 



FIG. 17. Female of Paper Nautilus Argonauta argo with 

 its brood-chamber shell, enveloped by the expanded ends 

 of two of the arms. 



the essential gonads, and are therefore bereft 

 of that useful criterion of a secondary sex- 

 character which has been discovered in 

 vertebrates. 



Let us consider, then, a few striking sex- 

 differences in their bearing upon Tandler's 

 theory. The female paper-nautilus, or ar- 

 gonaut, is very different from the male. She 

 is much larger, she has two " arms " peculiarly 

 modified to secrete a unique shell, not homo- 



