214 ENTO-PAEASITES AND HYGIENE. 



Let the possibility arise that those little animals become 

 rare for some reason, or that other circumstances make 

 it hard or actually impossible for the organism spoken 

 of to go on finding its food in the way followed hitherto. 

 It need not die out at once in consequence; being a lively 

 individual, it will easily surmount such difficulties and 

 try to make good the deficiency in some other manner. 

 It may go to other, perhaps bigger animals, trying to 

 take from them whatever it can get, in order to supply 

 its need of food. 01: course it cannot kill these bigger, 

 stronger creatures, nor eat them up entirely like its 

 former prey; it must be contented to tear away from 

 them single parts of their bodies — it has become an oc- 

 casional, a temporary parasite. This new mode of get- 

 ting food may be found even more advantageous than 

 the old one; the little creature has ''tasted blood" and 

 goes about in the same way whenever there is another 

 opportunity, without, however, carrying through this 

 mode of living consistently. They will in the majority 

 of cases fall back on their old way. After many genera- 

 tions it may result that the parasitic way of getting food 

 is practised oftener and more constantly; while it was the 

 exception formerly, it becomes the rule now; the going 

 back to the original mode will be the exception. A very 

 good example of this state of things is the leech, which 

 formerly fed on shells and snails, but has in the course 

 of time become a regular parasite, living on the blood of 

 frogs and fishes. Should it happen that the latter are 

 missing, it returns to its former habit of devouring the 

 above evertebrates. 



This gradual adaptation to parasitism will bring about 

 also a definite relation to the new host. In the begin- 

 ning the latter will not have been chosen deliberately, 

 but taken just as change brought it along. By and by a 

 certain selection will take place ; one animal is liked es- 

 pecially by our candidate for parasitism, or circum- 



152 



