it is sometimes convenient to have another separation between the phylum- 

 division and the class-division. So we have two or more "sub-phyla." There 

 may also be "sub-classes." In fact, we may make a sub-division wherever 

 we find it convenient, or wherever the material is sufficient in amount and 

 variety. For we need not suppose that a "class" — like bird or fish or i?isect, 

 for example — exists and merely waits for us to recognize it. In a sense, all 

 our sorting is artificial, although it is based on facts that we can actually 

 observe in natural objects. 



The "classes" have been broken down into "orders," and these into 

 "families." Within the families are the genera (singular, genus), each with 

 a variable number of species. As in the case of the species themselves, each 

 of these divisions is determined by the resemblances and differences that we 

 can observe. There can be no rule as to how much difference it takes to set 

 up a new species, or how many species should go into a genus. New species 

 are constantly being described, and older groups are constantly being re- 

 combined. 



A General Scheme The names we give to the main divisions and sub- 

 divisions in our schemes of classifying organisms are arbitrary or conven- 

 tional. It is nevertheless well to use them in the special senses of the 

 taxonomists instead of the informal everyday sense. Thus we speak of the 

 cat family, the dog family, the class birds, the order butterfly, the phylum 

 chordates, and so on. 



Since we sort according to physical characteristics, we naturally cannot use 

 the same basis for classifying plants and animals. Linnaeus classified plants 

 primarily on the flowers and other structures associated with reproduction. 

 He classified animals chiefly on the more obvious structural characteristics 

 and on their modes of locomotion and food-getting. Among both plants 

 and animals, however, the successive subdivisions are given the same names 

 (see pages 40 and 41). 



Using Classification^ No person can ever know all the plants or all 

 the animals. By observing and comparing different species, an individual 

 could in a lifetime learn to know several thousands of species by name. At 

 the same time, he could learn to recognize at a glance the class, order or 

 family in which to place many thousands of other species that he had never 

 seen before. This is not as difficult or mysterious as it sounds, for everyone 

 does just that every day without much effort. Suppose you see a kind of 

 "animal" that you have never seen before. You recognize it at once as a 

 "kind of bird" (class). Or you might say offhand, "That is a kind of parrot" 

 (order) or "a kind of woodpecker" (family). You might not guess that 

 the peacock is classed as in the "same family" as the common fowl, but you 

 would guess that the duck and the goose are "related". 



^See Nos. 2 and 3, p. 44. 

 39 



