36 GENERAL PRINCIPLES OF ZOOLOGY. 



are to be regarded as good species or as varieties of a 

 species. Zoologists are guided rather in practice by a 

 certain tact for systemization, which, however, in difficult 

 cases leaves them in the lurch, and thus the opinions of 

 various investigators vary. 



Change of Varieties into Species. The conditions 

 above discussed find their natural explanation in the as- 

 sumption that sharp distinctions between species and variety 

 do not exist ; that species arc varieties wliicJi have become 

 constant, and varieties are incipient species. The meaning 

 of the above can be made clear by explanation of a concrete 

 case. Individuals of a species begin to vary, i.e., compared 

 with one another they attain a great difference in character. 

 So long as the extreme differences are bridged by transi- 

 tional forms we speak of varieties of a species ; if, on the 

 other hand, the intermediate transitions have died out, and 

 the differences have in the course of a long space of time 

 become fixed, and so very much intensified that a sexual 

 union of the extreme forms results either in complete 

 sterility or at least in a tendency towards sterility, then 

 we speak of different species. 



Species may be Related to each other in Unequal 

 Degrees. In favor of this view, that varieties will in longer 

 duration become species, speaks also the great agreement 

 which in the large majority of cases exists between the 

 two. In the case of genera which comprise a remarkable 

 number of species, the species usually show also many 

 varieties ; the species are then usually grouped in sub- 

 genera, i.e., they are related to each other in unequal 

 degrees, since they form small groups arranged around 

 certain species. In regard to the varieties also the case is 

 similar. In such genera the formation of species is in 

 active progress; but each species formation presupposes a 

 high degree of variability. 



Phylogeny. It is now clear that the same argument 

 which has here been worked out in the case of the species 

 must also apply to the other categories of the system. 

 Just as by divergent development varieties become species, 



