Si2 MANUAL OF ELEMENTARY ZOOLOGY 



fying them. A more practicable basis is found in the fact 

 that animals which are closely alike will breed together and 

 give fertile offspring, whereas those which are less alike 

 will not. Thus the offspring of two horses is fertile, but 

 that of a horse and an ass is not, while breeding between 

 horses and oxen is impossible. The primary groups of 

 zoological classification consist of individuals which will 

 breed together to give fertile offspring, or of which it is 

 concluded from their likeness that they could do so. Such 

 a group is known as a species. We have seen examples of 

 the kind of differences which separate species in the case of 

 the hares and rabbits (p. 419), of the crayfishes (p. 239), of 

 the Hydras (p. 161), and of the Entamceba (p. 145). It is 

 believed that all the members of a species are united by 

 blood kinship ; that is to say, that they are all in the long- 

 run the descendants of one pair or several related pairs of 

 parents, so that their relationship is only an extension of 

 that which exists between offspring of the same parents. 

 Thus the resemblance between the members of a species 

 depends on two things : (1) their community of descent, 

 (2) their inability to weaken their likeness by interbreeding 

 with unlike kinds of animals and thus acquiring new 

 features. At the same time it must not be overlooked 

 that upon the average, two members of a species differ in 

 more respects than two children of one parent. 



Species are grouped together by zoologists into divisions 



of a higher grade known as genera. A genus 

 Groups. consists of several species which resemble one 



another closely, but its limits are determined 

 by convenience only, and are not natural, like those of 

 a species. To every species there is assigned a Latin 

 name consisting of two words, of which the first denotes 

 the genus to which the species belongs, while the second 

 is peculiar to the species. Thus the generic name of the 

 rabbits and hares is Lepus, the specific name of the rabbit is 

 cuniculus, the common hare is Lepus timidus, the mountain 

 hare L. variabilis. The names of the species of Astacus, 

 Hydra and Entamicba have already been given. The 

 Latin names of many species are arbitrary, and some are 

 even misleading, but they have the advantage of providing 

 a generally recognised, international nomenclature. In the 



