108 GENERAL ZOOLOGY 



They often refer to some striking characteristic of the ani- 

 mal ; thus, Melanoplus means "black armor," in allusion 

 to the dark-colored exoskeleton. Sometimes the reference 

 is to the locality where the animal is found, as atlanis, re- 

 ferring to the Atlantic states ; sometimes the name is given 

 in honor of some student of animals, as Darwin'ii, named 

 after the naturalist, Charles Darwin. The scientific name 

 first given to an animal, if accompanied by a description, 

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

 that name wherever found. This system of nomenclature 

 was introduced by Linnaeus. 



The words "species," "genus," and the other terms 

 applied to groups of animals are man's invention, for his 

 convenience in scientific description. Individuals which re- 

 semble 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 in- 

 variably 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, living under diverse 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 swallowtail 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, 



