628 LIFE : OUTLINES OF GENERAL BIOLOGY 



is likely to be a psychical factor that actually counts. On the tenta- 

 tive or experimental side, room must be left for novel answers-back 

 to the external periodicities, such as those of the seasons, and for 

 novel expressions of the internally rhythmic impulses. 



(VII) TROPISMS.—The next lower grade, where the psychical 

 aspect fades still farther, is that of obligatory movements — tropisms. 

 By a tropism is meant an engrained constitutional obligation to 

 adjust the body relativ^ely to the direction of the incident stimulus 

 so that the two sides — it may be the two eyes, ears, nostrils, antenna, 

 and so forth — are equally stimulated. It is not a deliberate balancing ; 

 it is an automatic means of securing physiological equilibrium. 

 When young eels or elvers are making their way up a river, they 

 automatically adjust themselves so that both sides of their body 

 are equally affected by the pressure of the stream. And this tropism 

 is sufficient to account for a great part of their apparently deter- 

 mined swimming against the current and straight ahead, even in 

 rapids. What happens in an eddy we do not know. 



When a moth flies past a candle it has one eye much more illu- 

 mined than the other, and it automatically adjusts its body so that 

 equilibrium of stimulus is attained. It bends round so that the two 

 eyes are equally illumined, and if it continues flying quickly in thi 

 same direction, it must fly into the flame. If it is flying slowly, the 

 reaction against great heat may prove stronger than the tropism in 

 relation to light. Or if the moth turns outwards at the critical moment . 

 both eyes thus becoming equally unillumined, then it will be saft- 

 from the candle for the time being. 



An interesting fact is the not infrequent reversal of tropism^ 

 when a certain limit is crossed; or when there is a notable chanp« 

 in the environment, or in the animal's physiological condition. 

 Some animals, like scorpions and crayfishes, that are constitution- 

 ally light-shunning (negatively heliotropic) are unable to keep away 

 from an unusually bright light. The little crustaceans called Gam- 

 marids, that are common in brooks, are constitutionally light- 

 shunning and given to hiding under stones; but if a few drops of 

 acid be added to the water of the aquarium in which they are living, 

 they become positively heliotropic! Some caterpillars are constitu- 

 tionally wound-up to climb higher and higher on their food-plant 

 (negatively geotropic), but when they reach full size and their 

 physiological state changes, their "forced movement" or tropism 

 reverses, and they become as bent on going down as they previously 

 were on climbing up. It is also the case that a tropism may be inter- 

 rupted by another tropism, or by individual initiative. The obligatory 

 character of tropisms must not be exaggerated, but in the ordinary 

 routine of the animal's life it is just the obligatoriness that makes 

 them so profitable. In ninety-nine cases out of a hundred they work 



