CHAPTER XIV. 



CLASSIFICATION. THE DIFFERENCES BETWEEN 

 PLANTS AND ANIMALS. 



In a footnote to p. i, it was promised to define what naturalists 

 mean by the terms " genus " and " species." A species is a group 

 of animals (or plants) of which the individuals agree in all but 

 perhaps the very minutest details. This may be illustrated by 

 common examples among the earthworms. Under stones and 

 in damp places there is, frequently to be met with an earthworm 

 of a greenish colour and small size ; it is commonly coiled into 

 a crescent, and is somewhat sluggish in habit. On wet morn- 

 ings, and in dryer weather by digging, a different form, of a 

 greyish blue colour and more active habits, is to be found. 

 The green species is known as Allolobophora chlorotica, the 

 bluish worm as Allolobophora cyanea. On examination the green 

 species will be found to have a clitellum occupying segments 

 30-37, sometimes including also the twenty-ninth; it has, more- 

 over, three pairs of spermathecas. The bluish form has a clitel- 

 lum occupying only segments 29-34, and there are but two 

 pairs of spermathecae. Wherever we find an earthworm with the 

 characters just given, it will be found to agree in all other par- 

 ticulars with other individuals presenting those same characters. 

 Some examples of A. chlorotica do not show the usually 

 characteristic green colour, and in some the clitellum begins a 

 segment earlier than is usual for the species, i^. segment 29. 

 Some naturalists separate off those individuals which show 

 these minute differences from the typical form into a variety or 

 sub-species; but it is simplest merely to speak of such ex- 

 amples as variations from the type form. It is not possible for 



