WHAT IS MEANT BY "A SPECIES"? 99 



double names we owe to the great Swedish naturalist of 

 the eighteenth century, Linnaeus. He proposed also that 

 the relationships of living things to one another should be 

 further expressed by grouping like genera into " families," 

 then like families into "orders," and like orders into 

 "classes." And since his day we go further and group 

 classes into "phyla" or great stems of the animal pedigree. 

 In this way a complete hierarchy or system of less and 

 more comprehensive groups has been established, and is 

 the means by which we indicate the natural groups of the 

 family-trees of plants and of animals, what, in fact, is 

 called the " classification " of each of these great series of 

 living things. Linnaeus compared his system of groups to 

 the subdivisions of two armies. Thus, the one army 

 represents the whole animal series, the other the whole 

 vegetable series. An army is divided into (i) "legions," 

 these into (2) " divisions," " divisions " into (3) " regiments," 

 regiments into (4) battalions, and battalions consist of 

 (5) companies, consisting of individual soldiers. Accord- 

 ing to Linnaeus, we may compare the legions to 

 classes, which are divided into orders, comparable to 

 divisions ; these into families, comparable to regiments ; 

 these into genera, comparable to battalions; and these 

 into species, comparable to companies, or ultimate 

 groups of individual units or soldiers. 



Just as the legions, divisions, regiments, battalions 

 and companies of an army have each their own name or 

 at any rate a distinctive numeral assigned to them in 

 order that they may be cited and directed, so are names 

 given to each class, order, family, genus and species of 

 the classification or enumeration of the kinds of animals 

 and plants. Here, for instance, are the names of the 

 greater and smaller groups in which our common " white " 

 finds itself enrolled. Class Insects. Order Lepidoptera. 

 Family Pieridae. Genus Pieris. Species brassicae. 



