62 ZOOLOGY 



with one another and produce fertile offspring. When 

 members of two different species are forcibly crossed with one 

 another, in most cases no offspring at all are produced; in 

 a few cases some young are produced which are themselves in- 

 capable of reproduction. Such young are termed hybrids ; a 

 familiar example of these is the mule, which is the result of a 

 cross between the horse and the ass. In the very rare cases 

 where the hybrids are capable of reproduction they exhibit an 

 unstable heredity, and their offspring tend to revert to the 

 father or mother species. 



Whilst^ so far as we know, the species is the same thing in 

 whatever group of the animal kingdom it is found, this is not 

 true of the higher divisions of the system. A genus of reptiles 

 is a different thing from a genus of singing-birds. In fact all 

 the higher denominations are merely names for more or less 

 convenient bundles of species which resemble one another more 

 or less closely, and the number of grades which we require in 

 classifying a group of animals depends on the thoroughness of 

 our analysis. It is often necessary to introduce intermediate 

 grades of classification between order and tribe, family and 

 genus, and so on ; this is done by introducing the prefix sub-. 

 Thus we speak of sub-genus, sub-order, &c., but these grades are 

 only used if the species within a genus, for instance, fall into 

 groups, the members of which more closely resemble one 

 another than they do the other species belonging to the same 

 genus ; and often what one authority calls a sub-genus, another 

 makes a genus, and so on. What the meaning of the system , 

 of classification may be, in a word, what is the origin of 

 species? is one of the most profound and most interesting 

 questions in zoology, and this we shall discuss in the next 

 chapter. 



CHAPTER VI 



THE ORIGIN OF SPECIES. 



WE have just learnt in the last chapter that the species is the 

 real basis of classification, and that higher grades of classifica- 

 tion are only bundles of species. The question instantly occurs 



