204 PROCEEDINGS OF THE ACADEMY OF [April, 



specialization between them ; in the one case the units are indi- 

 viduals, having physical disconnection, while in the other they are 

 not individuals, but parts of one whole. In the former they com- 

 pose not a perceptible but a thinkable whole, which we term a 

 species. 



The question is, accordingly, where the individuals of a species 

 are heteromorphic, on which of them should the classification be 

 based ? On the males or on the females, on the sexual or the 

 asexual individuals ? Our classification must start Avith the single 

 individual, and proceed next to an arrangement of the individuals 

 into those lowest groups known as species. If all the individuals of 

 a given generation of a species are monomorphic, the consideration 

 need not pass beyond any one individual, for each individual would 

 show the full characteristics of the species. And since this is not 

 the case in species in which the indiA^duals are heteromorphic, in 

 such species all the component individuals must be studied before 

 the species can be mentally defined. In other words, while indi- 

 viduals are the primary materials of classification, species are the 

 primary group units and so must be defined as to include all the 

 kinds of individuals composing them. 



Starting then with the principle that individuals ai'e to be first 

 considered, and that a classification of the most primary groups — 

 i. e., species — demands a knowledge of all the individuals which 

 compose such species, Ave must not in classification neglect the con- 

 sideration of any individual. Therefore classification must be based 

 upon all individuals, since otherAvise our concepts of the loAA'est 

 group Avould be incomplete. 



To the systematist are known many instances of species Avith 

 sexual dimorphism, Avhere the indiA'iduals of one sex are less valua- 

 ble than those of the other Avith regard to closely related species. 

 Thus among the insects, the females of Disptera are, as a rule, 

 difficult to distinguish specifically, Avhile in the less conservative 

 males specific characters are much more clearly pronounced, and 

 in this particular case much of the classification has been based 

 upon the genital armature of the male.^ In birds also the males 

 generally show greater differences than the females ; this is observed 



*If the genital armature of the female, which is to greater or less 

 extent adapted to that of the male, be examined more thoroughly than 

 it has at present, perhaps it, too, would afford good specific distinctious. 



