XVIU INTRODUCTION. 



life of a great nobleman ; and it is not rare to find, by 

 the side of the humble pickpocket, the audacious 

 brigand of the high road, who lives solely on blood and 

 carnage. A great proportion of these creatures always 

 escape, either by cunning, by audacity, or by superior 

 villany, from social retribution. 



But side by side with these independent existences, 

 there are a certain number which, without being para- 

 sites, cannot live without assistance, and which demand 

 from their neighbours, sometimes only a resting-place 

 in order to fish by their side, sometimes a place at their 

 table, that they may partake with them of their daily 

 food ; we find some every day which used to be con- 

 sidered parasites, yet which by no means live at the 

 expense of their hosts. 



When a copepode crustacean instals himself in the 

 pantry of an ascidian, and filches from him some dainty 

 morsel, as it passes by ; when a benevolent animal 

 renders some service to his neighbour, either by keeping 

 his rack clean, or removing detritus which clogs certain 

 organs, this crustacean or this animal is no more a para- 

 site than is he who cowers by the side of a vigilant and 

 skilful neighbour, quietly takes his siesta, and is con- 

 tented with the fragments which fall from the jaws of 

 his companion. We may say the same thing of the fish 

 which, through idleness, attaches itself, like the remora, 

 to a neighbour who swims well, and fishes by his side 

 without fatiguing his own fins. 



The services of many of these are rewarded either in 

 protection or in kind, and mutuality can well be exercised 

 at the same time as Jiosjntality, 



Those creatures which merit the name of parasites 



