HAVE FISHES MEMOKY. 379 



the investigation we need not consider to what extent the stimuhis is 

 not only felt and responded to, but perceived. As is well known, 

 even man, equipped as he is with a very acute capacity of perceiving, 

 often responds to stimuli which he does not perceive, if his attention 

 is not particularly directed to them. In fact, he can not recognize all 

 the stimuli that excite him. In the course of the following investiga- 

 tion, perceptions will be spoken of only in cases when their presence 

 is capable of being distinctly proved. 



Another reservation must be made before we examine the sensations 

 attained b}^ way of the system of nerves. In the world of animals, as 

 well as in the world of plants, there is a series of phenomena obviously 

 independent of the intervention of a nerve apparatus. Not onlv 

 plants, but also animals show the effects of heliotropism (phototrop- 

 ism) — that is, the tendency to turn toward or away from the light. 

 These effects appear even in animals in which not a vestige of a 

 nervous system can be demonstrated to exist. Similar phenomena, 

 common to the lower animals and plants, are known in connection 

 with heat, with chemical agents of irritation, with the tendency of 

 bodies to seek a state of equilibrium. We have not yet discovered 

 the mechanism or the disposition of the plasma at the root of these 

 phenomena which can be evoked and which disappear with the same 

 conformity to law as, let us say, the motion of iron filings toward a 

 magnet. These various "tropisms" are of widespread occurrence, 

 and their influence upon the general condition of organic beings has 

 been carefully studied. The upper limit of their occurrence, in the 

 ascending scale of animal orders, has not been determined. But we 

 have no reason for supposing that the "sporting of the merry little 

 fish in the sunlight" involves other processes than the mounting of 

 the larvffi of lower marine animals to the sunny surface of the water, 

 or the conduct of certain bacteria that constantly seek the part of their 

 habitation exposed to the light. The relation of these primar}^ biologic 

 forms to light is as characteristic and as much subject to law as that 

 of iron to the magnet. 



The conduct of the youngest brood of fish, still attached to the yoke- 

 sac as they swim about, is doubtless regulated hy law with regard to 

 light, the heat of the surrounding medium, and pro])ably many other 

 conditions of the outer world; it is born with them — rooted in their 

 organism. Most probably the phenomenon usually called "flight" 

 should be classed among "tropisms." It is present at a time at which 

 a developed nervous system is out of the question. Another accom- 

 plishment existing at birth is the coordination of many movements. 

 They depend as much upon the structure of the nmscles as upon that 

 of the nervous system. Schaper destroyed the nervous system of 

 frog eggs in the early stage of development. When they were older, 

 he saw them swim about, although subsequent investigation showed 



