1/4 Elementary Zoology. 



the careful observer to confound these two species of Allolo- 

 bophora with each other or with any other earthworms. The 

 two forms have a real existence as definable animals. We 

 may go a step further and complete the definition of the 

 species by stating that they both inhabit the greater part of 

 Europe. The area inhabited by a species is just as much a 

 part of its definition as is its colour, or shape, or internal 

 structure. 



Now, each of these worms has two names — Allolobophora 

 and chlorofica, Allolobophora and cyanea. The second name is 

 the specific name, the first the generic. The two species, and 

 about sixty others, agree with each other to form a larger 

 assemblage of earthworms than a species, to which the term 

 " genus " is applied. All the species of Allolobophora agree in 

 having the male pores upon the fifteenth segment, in which 

 they contrast with another but smaller assemblage of worms in 

 which the same pores are situated upon the thirteenth segment. 

 This distinction is held by systematists to be of more import- 

 ance than variations in the number of spermathecse, position of 

 clitellum, etc. ; and, accordingly, the worms with male pores 

 upon the thirteenth segment are separated oif into a genus, 

 Alltirus. But while there can be no difference of opinion about 

 the existence of a given species — save as to the use of such terms 

 of description as " sub-species " and " variety " — a genus has no 

 such definite existence. The animal world consists of so many 

 ■ species which are grouped into fewer genera. The aim in the 

 formation of genera is to indicate real affinities, genuine blood 

 relationship. Such relationships, however, can only be sur- 

 mised, they cannot be proved ; hence notions as to what 

 coristitutes a genus must and do vary with the individual 

 naturalist, for the " personal equation " comes into play. 



Genera, again, are grouped into larger divisions, termed 

 "families." The same remark that was made about genera 

 may be also made about families. One naturalist will decline 

 to regard as more than a genus what another will consider 

 entitled to family rank. But both believed, of course, that the 

 family consists of genera which are more nearly allied to each 

 other than any one of them is to a genus belonging to another 



