THE WEB OF LIFE 305 



catulus or minutus ; if it enters another species it is un- 

 successful. Yet the same larva in some other countries 

 is able to continue its Ufe in other species of water-snail ! 



When an examination is made of the food- canal of a 

 bird or mammal or other Vertebrate, which has not been 

 previously studied, some new species of parasite is very 

 generally found. There is a remarkable individuality in 

 the parasitic infection of distinct types. This probably 

 illustrates the role of isolation in assisting the formation of 

 species. Just as there is an Orkney vole and a St. Kilda 

 wren, and a distinct species of snail in each valley in Hawaii, 

 so there are dif!erent tapeworms, flukes and threadworms 

 in diverse hosts. In some cases, where the species of host 

 are nearly related, the species of parasites seem also closely 

 akin, and it should be asked in such cases whether the 

 observed differences in the parasites are really fixed hered- 

 itary characters, and not individually acquired features 

 induced by the shght pecuKarities of environment. 



There are curious little Crustaceans, called Lamippids, 

 of the order of Copepods, which occur, for instance, in 

 burrows among the spicules of Alcyonarian corals. They 

 are very distinctive httle creatures, characteristic in their 

 buccal armature at one end and in their caudal fork at 

 the other. They have been studied systematically by 

 A. de Zulueta, who finds that each species of Lamijype 

 has its particular host. Some hosts may harbour two or 

 three species, but no species occurs on two hosts. It would 

 be interesting to transfer some young Lamippids from their 

 proper host to another, to see whether some of the alleged 

 specific difierences are not directly due to the immediate 

 environment. Some of the species may be what we venture 

 to call ' modification species '. 



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