96 GENERAL ZOOLOGY 



name first given to an animal, if accompanied by a descrip- 

 tion, is the name it must bear, and the species is known under 

 that name wherever found. This system of nomenclature was 

 introduced by Linnseus (see p. 443). 



We must not overlook the fact that in the study of animals 

 we have to deal only with individuals. The words " species," 

 " genus," and the other words used beyond, are man's inven- 

 tion, for his convenience in scientific description. Individuals 

 which resemble each other in a large number of characters 

 and especially if the individuals are able to interbreed are 

 usually said to belong to the same species. The test of inter- 

 breeding, while of almost universal application, is not invari- 

 ably a means of distinguishing the species, since in some 

 cases two different species can produce offspring (called 

 hybrids), though the latter are usually not fertile, that is, 

 they are not themselves capable of producing young. 



Often within the limits of a single species there are groups 

 of individuals which vary from the others in one or more 

 characters. Especially is this true of those species with a 

 wide range, including different climatic conditions. In such 

 cases the different forms which the species assumes are termed 

 varieties, and a varietal name is sometimes added to the 

 generic and specific names. We have already referred to the 

 seasonal variations of the black swallow-tail butterfly. 



The different genera are arranged in groups, or classified, 

 according to their resemblances and differences. A number of 

 genera which show similar structural characteristics of more 

 general character than those used to constitute a genus make 

 up a family. Thus the locusts, not only of the genus Melano- 

 plm but also of all the other genera found in North Amer- 

 ica, together with the genera of the Old World, have short 

 antennae, in common with other characteristics, which cause 

 them to be placed in the locust family, Acrid'idce. Similarly, 

 the katydid and other green grasshoppers, with many meadow 



