XXVI INTRODUCTION. 



Tinder the names of sub-kingdoms, branches, or 

 departments, namely, — 



Vertebrata, 



MOLLUSCA. AnNULOSA. 



Ccelenterata, 

 Protozoa. 



Sub-kingdoms are divided into classes, classes 

 into orders, orders into families, families into 

 genera, and genera into species. It is usual to 

 state that, "the species is a living form represented 

 by individual beings, which reappears in the pro- 

 duct of generation with certain invariable cha- 

 racters, and is constantly reproduced by the ge- 

 nerative act of similar individuals."^ Bat it 

 cannot be said that this definition is altoo-ether 

 unimpeachable. 



It sometimes happens that between different 

 individuals, known to belong to the same species^ 

 certain distinctive peculiarities are observable. 

 When such peculiarities are strongly marked, the 

 individuals which possess them are denominated 

 varieties. Permanent varieties, i, e. those which 

 always produce offspring similar to themselves, 

 are distinguished by the name of races. 



The question how far individuals belonging to 

 the same species may vary is intimately con- 



