GENERAL PRINCIPLES OF ZOOLOGICAL CLASSIFICATION n 



same species, and when so great that we must place them in 

 distinct species? To answer the second question first, the 

 generally accepted rule is that if two organisms will interbreed 

 and produce fertile offspring they belong to the same species. 

 There are a number of animals which will interbreed and pro 

 duce sterile offspring, especially under domestication ; such off- 

 spring are known as hybrids. The mule, the result of crossing 

 the mare with the he-ass, is a familiar example ; the dog will 

 cross with the wolf, jackal, and fox ; the sheep, with the goat 

 and the pig ; hares with rabbits, various species of birds with 

 one another, and similar instances occur in the lower animals 

 and in plants as well. In a few cases hybrids have been known 

 to reproduce, so that while the rule holds in general it is not 

 absolute. These facts are of great importance, as we shall see 

 in a later chapter when we come to consider the origin of 

 species together with other problems in theoretical biology. 



Considering, then, animals which freely interbreed and pro- 

 duce fertile offspring as belonging to the same species, we note 

 in many cases certain characteristics constantly present in some 

 individuals and absent in others ; these characteristics are 

 capable of being transmitted to the offspring, and thus we have 

 varieties ; familiar examples are the setter and the terrier among 

 dogs, the fantail and the tumbler among pigeons. The off- 

 spring which result from the crossing of varieties are known as 

 mongrels, and they are always fertile, with a few exceptions 

 among plants. Members of the same species are always fertile 

 except after prolonged interbreeding, when the offspring of a 

 single pair are continually chosen for reproduction ; in such 

 cases absolute sterility has been obtained in some plants, and 

 the same law doubtless holds for animal species. 



In a complete study of plants and animals we have to con- 

 sider, not only organisms which live at present on the earth, but 

 others which have lived in the past and may or may not have 

 become extinct. For this study a pretty complete knowledge of 

 existing plants and animals is desirable and also some acquaint- 

 ance with geology as well ; but in taking up the classification of 

 animals it is frequently convenient to refer to representatives of 

 various groups which have existed in the past history of the earth, 

 and so we may note here one or two facts which properly belong 



