2 ANIMAL PAEASITES AND MESSMATES. 



under certain given circumstances, and do not usually 

 merit this appellation. 



In the higher animals, this relation between them is 

 generally well known, and justly appreciated, but it is not 

 the same in the inferior ranks ; and more than one 

 animal may pass for a messmate or a parasite, for a 

 robber or for a mendicant, according to the circum- 

 stances under which he is observed. The sharper passes 

 for an honest man as long as he has not been taken in 

 flagrante delicto. Thus, in order to be just, we must 

 carefully examine the indictment, and not pronounce 

 sentence without strict examination. 



The greater part of those animals which have estab- 

 lished themselves on each other, and live together on 

 a good understanding and without injury, are wrongly 

 classed as parasites by the generality of naturalists. 

 Now that the mutual relations of many of these are 

 better understood, we know many animals which unite 

 together to render each other mutual assistance ; while 

 there are others which live like paupers on the crumbs 

 which fall from the rich man's table. There are many 

 relations between the different species which can be dis- 

 covered only after minute examination, but which have 

 recently been appreciated with greater impartiality. 



Animal messmates are rather numerous, and com- 

 mensalism has been observed, not only in animals of the 

 present age, but in those of the primary epoch. Wyville 

 Thomson explained to me, while I was myself his mess- 

 mate at Edinburgh, at the meeting of the British Asso- 

 ciation in 1871, that the polyps of the Silurian age 

 already practised it. 'We do not class among animal 

 messmates those living creatures which, like the birds 



