APPENDIX D 227 



entirely in the present. To this it may be objected, however, 



that various insects display an instinctive memory, and, for 



instance, return again and again with food to the nest where 



they have laid their eggs. If, however, my definitions are correct, 



these returns are not due to memory, but to an impulse (similar 



to that which causes them in the absence of experience to know 



a fit spot wherein to lay their eggs), which causes them again and 



again to return to this particular place, quite independently of 



any recollection of having been there before. It has even been 



denied that animals so high in the scale as fish possess a memory 



(the power of acquiring mental traits). The seat of memory has 



been held to be the cortex of the brain, and fish alone of all 



vertebrata have no cortex. I think, however, there can be no 



doubt that fish have some power of acquiring mental traits, since 



trout in a much-fished stream soon grow more wary, Indeed, 



memory may be detected in animals much lower than the fish. 



Even so low in the scale as the oyster is a rudimentary capacity 



for mental acquirement observable, for " even the headless oyster 



seems to profit by experience, for Dicquemase asserts that oysters 



taken from a depth never uncovered by the sea open their shells, 



lose the water within, and perish, but oysters taken from the same 



place and depth, if kept in reservoirs, where they are occasionally 



left uncovered for a short time and are otherwise incommoded, 



learn to keep their shells shut, and then live for a much longer 



time when taken out of the water." 



As I have already said, speaking in general terms, the higher 

 placed an animal is in the scale of life the greater is its power of 

 acquiring mental characters, as will be apparent presently and as 

 might have been expected; but it is also true that the higher 

 species of a lower class or order often exhibit greater capacities 

 for acquirement than the lower species of a higher class or order. 

 It is even true that some invertebrates exhibit far greater mental 

 receptivity than many vertebrates. Speaking again in general 

 terms, the power of acquiring mental characters is only developed 

 to a considerable extent in such animals as tend their young, and 

 in them it is developed in proportion to the length of time 



