INDIKECT SELECTION. 361 



for there is no other and certainly no more convenient way. 

 This stream enters both the gill-cavities at once ; it is therefore 

 easy to understand that the larvae sometimes get into the right 

 and sometimes the left cavity, and also that in the course of 

 their growth, which is probably very rapid, they must hinder 

 other larvae from settling in the same spot, or perhaps they feed 

 on the later arrivals as an easy prey. But it is highly remark- 

 able that when one individual has established itself in one bran- 

 chial cavity it is impossible for another animal of the same 

 species to settle in the other unoccupied gill-cavity ; so at least 

 we may conclude from the fact that hitherto no case has been 

 described of the simultaneous occurrence of two individuals 

 in the gill-cavities of the same Crustacean. I myself, though 

 I have collected and studied hundreds of these animals, have 

 never met with one exception to this rule, and my observations 

 have been confirmed by so distinguished a student of the 

 natural history of Crustaceans as Professor Gerstaeoker.* So 

 far as I can see, there is only one possible way of exj)laining this 

 striking fact. The guest already in possession of one of the 

 cavities— let us say the left — cannot of course directly prey 

 upon or turn out the larvae brought by the current into the 

 other unoccupied cavity ; it can only do this indirectly, by so 

 influencing its host that after the establishment of the first in- 

 truder it is no longer fit for the reception of a second.'^^ 



Where, as in this case, the two different animals do not 

 come into actual contact and yet exert a definite influence on 

 each other, this under all circumstances can only be a selective 

 action, and this selection, as is well known, may l>e effected in a 

 variety of ways. With regard to the end I have in view, it 

 would be superfluous here to discuss all or even most of these 

 ways j the general result will be set in a full light by consider- 

 ing a small number of examples. But before we more closely 

 examine the means which nature has bestowed on animals to 

 give them the advantage in their relations towards others, it will 

 be desirable to say a few words in general consideration of those 

 mutual relations of any two animals which are called forth 



* The well-known editor of the section on Crustaceans in Bronn's 

 great work on General Zoology. 



