1902.] XATURAL SCIENCES OF PHILADELPHIA. 205 



also in niauy mammals, as the Cervidae, in the Gordiacea, and in 

 uumeroas other groups. Bui while a description of a new species 

 is in practice often based upon individuals of one sex, in cases 

 where those of the other sex have not been seen, yet such a descrip- 

 tion is only a partial characterization of the species, for the comple- 

 ment al individuals have been neglected; it is not, strictly speaking, 

 a description of a species, but of only one kind of individuals of 

 that species. Now, let us consider two species, A and B, each with 

 sexual dimorphism, the females of the two not appreciably different, 

 while the .males show marked differences. Then on comparison of 

 tlie females alone, one would conclude that they composed only one 

 species; of the males alone, that they composed two perfectly dis- 

 tinct speci&s. The error of basing the characterization upon either 

 sex by itself Is then ob\'ious, for A and B are neither one species, 

 nor are they c^ear^j/ distinguishable species ; the similarities of the 

 females of the two show that the species are very closely related, 

 while the differences of the males show that they are nevertheless 

 distinct. The species would then be characterized, " females indis- 

 tinguishable, males differing in such and such characters."^ 



Tills reasoning, based upon cases where neither sex is markedly 

 degenerate, may be, to a certain extent, a guide also for such cases 

 where either the male or the female is degenerate in comparison with 

 the other sex. Such cases are numerous, as the degenerate charac- 

 ter of the male in the Rotatoria, in the Echiurid Bonellia, the 

 marked sexual dimorphism in the Coccidse and some parasitic 

 Copepoda, etc. If in these the classification were based upon the 

 more degenerate individuals, the species would be ascribed too low 

 a phyletic rank, and if upon the less degenerate alone, too high a 

 rank. For here, as for the other cases, the characterization of a 

 species must be the sum total of the characterization of its differ- 

 ent individuals. In such a species the male and the female may 

 have had a corresponding ontogenetic development up to a certain 

 point, then the one developed regressively and the other progres- 

 sively, a difference in mode of life beiug the determining factor 



* In the particular case used for discussion we have excluded the pos- 

 sibility of the occurrence of two kinds of males coinplemental to a single 

 kind of female, i. e., dimorphism of tlie males. When females appear 

 indistinguishable, however, after a thorough study of all their parts, 

 while the males are easily distinguished, the classifier should disprove 

 that it is not a case of male dimorphism before he founds two species. 



