30 GENERAL PRINCIPLES OF ZOOLOGY. 



hybridization of different species, are injurious and are abhorred 

 by Nature. Sexual reproduction possesses an optimum; if this be 

 departed from in either direction, diminution gradually follows. 

 But for that reason it has already been said that here gradual and 

 not primary differences exist, and therefore this character cannot 

 be employed as a primary distinction* bet ween species and varieties. 



Difficulties in Classification. The final result of all this dis- 

 cussion may be summed up as follows: up to the present time, 

 neither by physiological nor by morphological evidence has there 

 been successfully fixed in a clear and generally applicable way a 

 criterion which can guide the systematist in deciding whether 

 certain series of forms are to be regarded as good species or as 

 varieties of a species. Zoologists are guided rather in practice by 

 a certain tact for classification, which, however, in difficult cases 

 leaves them in the lurch, and thus the opinions of various investi- 

 gators vary. 



Change of Varieties into Species. The conditions above dis- 

 cussed find their natural explanation in the assumption that sharp 

 distinctions between species and variety do not exist; that species 

 are varieties which have become constant, and varieties are incipient 

 species. The meaning of the above can be made clear by explana- 

 tion of a concrete case. Individuals of a species begin to vary, 

 i.e., compared with one another they attain a greater or less- 

 difference in character. So long as the extreme differences are 

 bridged by transitional 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 marked 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 time become 

 species, is the great agreement which in the large majority of 

 cases exists between the two. In genera which comprise a remark- 

 able 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 presup- 

 poses a high degree of variability. 



