40 Introduction to Animal JSIorpJiology. 



mensalism, where one animal lives on and feeds with 

 its host, but does not nourish itself at its expense. A 

 commensal may be merely indebted to its host for 

 house-room, as the Coronulse on the Whale's skin 

 (Oikosites), or it may feed on the food of its host, and 

 then is a Koinosite. This is called pseudoparasi- 

 tism. 2nd. A true parasite feeds on the tissues or 

 juices of its host, and may be an ecto- or an endo- 

 parasite, according as it is attached to the outside or 

 to the inside of its body surface. 



In classification it is well to bear in mind that 

 no character of structure or form is of equivalent 

 taxonomic value in all tribes, but the value of each 

 varies in each class [Forbes] ; also that a character is of 

 good value in classification when its presence enables 

 us to predict correlative but independent characters 

 as accompanying it [Rollcston]. 



The origin of species is a favourite subject of theory, 

 on which modern opinions may be ranged in two 

 series — ist. The special creation hypothesis (whose 

 advocates differ as to the number of separately created 

 types, their opinions depending on their conceptions 

 of species) ; and 2nd. The evolution theory, that dif- 

 ferent forms are descended from common stocks. The 

 supporters of this view differ according as they attach 

 a greater or lesser importance to the different physical 

 causes of variation. The fact that scarcely two indi- 

 viduals are alike may be stated generally thus, that 

 the descendants of a common stock are liable to vary. 

 As a larger number of individuals are produced than 

 can continue to exist under the conditions of food, 

 temperature, and other surroundings, some only, and 

 those necessarily the fittest, will survive; the fittest 



