in] THE FACTORS OF DISTRIBUTION 85 



of optimal conditions of such young plaice, it will very 

 soon be inhabited by a multitude of fish feeding 

 greedily on the nutritious shell-fish. Now we must 

 assume that they actually 'like' the mussels, for it is 

 the case that even if other food is abundant the plaice 

 of certain regions will take the young mussels to the 

 exclusion of other shell-fish there must therefore be 

 choice on the part of the fish. A fisherman will 

 doubtless say that the bed of mussels has ' attracted ' 

 the plaice, and a physiologist of the mechanistic school 

 will explain the aggregation of the fish on the shell-fish 

 bed in terms of his ingenuous theory of tropisms, and 

 may call it an example of 'trophotaxis,' or some such 

 term. What doubtless happens is that the fish, 

 moving about in all directions within their optimal 

 area, accidentally encounter the shell-fish bed and 

 finding food which is to their liking they will stay 

 there. While feeding upon the mussels they still 

 move about and it must often happen that a fish will 

 leave the bed and so will not obtain so much food. 

 But by turning about it will again, after a number of 

 trials and failures, re-enter the area of abundant food. 

 In attempting to explain the movements of a marine 

 animal we assume the existence of consciousness in 

 it, for the same reasons as we have to assume it in 

 other individuals of our species ; and it seems sound 

 enough to assume that in the fish as in ourselves con- 

 sciousness has become an effective factor of evolution 



