84 BIOLOGICAL LECTURES. 



often, however, and trying to swim through the meshes of the 

 wire screen back to his habitual haunt. As soon as the fish is 

 at the sunny end, I place across the aquarium, between him and 

 the shady end, a piece of wire screening, impervious save at the 

 upper right-hand corner, where there is a hole 2x3 inches (my 

 aquarium was 4 feet long, 2 feet wide, the space in which the 

 fish was now confined being 2 feet long, 10 inches wide, and 10 

 inches deep), and remove the screen used to push him down to 

 the sunny end. The situation is, of course, " sight of shady 

 end, feeling of the sunlight and of the walls around and screen- 

 ing in front." The animal reacts by remaining still, swimming 

 around, swimming particularly up and down the screening 

 which separates him from the shady end, poking his head 

 against it repeatedly in attempts to swim to the shady end. 

 The swimming is, for the most part, along the bottom of the 

 aquarium, but occasionally the fish rises slowly up and pokes 

 his head against the upper part of the screen. If he does this 

 at the right-hand end, he of course pokes through the hole 

 and gets to the shady end. Sooner or later he is practically 

 sure to do so. If, after giving him fifteen minutes' enjoyment 

 of his preferred habitat, you repeat the experiment, and so 

 again and again, the remaining still and swimming back and 

 forth and poking the head against the bottom of the screening 

 will gradually decrease, until finally the fish, when confronted 

 with the situation, will go directly to the right end, rise up, 

 and glide through the hole without more ado. Moreover, we 

 shall find, when pushing him down with the first screen, that 

 he no longer makes the useless attempts to swim through the 

 screen, which only hurt his head, but swims down to the sunny 

 end as soon as the screen is put behind him. Experiments 

 with different sorts of obstacles give the same results. Evi- 

 dently the presence of a cerebral cortex is not a prerequisite to 

 the formation of these associations. 



How far down in the invertebrate series this process exists 

 remains a question for observation and experiment. Any ani- 

 mal which finds its way to feeding grounds or home apart from 

 direct response to sense-impressions would seem to do so by 

 virtue of associations thus formed. 



