30 ENVIRONMENTAL RELATIONS 



which has caused it to run away, for it thereby acquires the habit of 

 running away. In the lower animals we find the same condition; their 

 behavior may be modified, but the modifications are less permanent 

 than in man and other mammals. 



Changes within the organism cause approximately fixed environ- 

 mental conditions to act as stimuli. Changes are going on all the time 

 within the organism. Such changes may result from growth, maturation 

 of sexual products, or other causes. The organism may be in physiologi- 

 cal equilibrium (see p. 26) in a given set of conditions before the devel- 

 opment of the eggs and spermatozoa begins, but these processes are 

 accompanied by other great physiological changes, which frequently put 

 the animal out of adjustment to its surroundings. 



The queen ant is in physiological equilibrium in the darkness of the 

 nest until she becomes sexually mature. She then becomes positively 

 phototactic, goes toward the light, flies from the nest with the males, 

 and, being negatively geotactic, stays away from the ground. When fer- 

 tilized, she at once becomes negatively phototactic, positively geotactic, 

 and positively thigmotactic. Accordingly she places her body in contact 

 with the ground, and burrows into it and starts a new colony. The ant 

 is then in a different physiological state after becoming sexually mature, 

 and in a third state after fertilization. 



3. ENVIRONMENTAL CHANGES 



a) Daily changes. The physiological responses of animals to these 

 changes have an important bearing on their relation to each other. 

 Some forms are diurnal, others nocturnal, others crepuscular. Some are 

 probably active all the time, but move into different positions in the day 

 and in the night. For example, some pelagic animals (which float or 

 swim freely in water and are independent of bottom) are numerous near 

 the surface at night, but migrate to considerable depths during the day, 

 as a response to light. Many animals bear relations to day and night, 

 which in some cases may be of an adaptive character. Some forms are 

 active during the day, and hide themselves during the night, either in 

 burrows or under suitable objects. Those which simply crawl under 

 loose objects during the day frequently appear at artificial lights in 

 the evening. 



It is not impossible that there are structures in the bodies of many 

 animals which are the product of the different conditions of day and night 

 during critical periods of growth. Thus Riddle (46) has found that the 

 barrings of the feathers of certain birds are due to low blood pressure at 

 night (feeble circulation) during the growth of the feathers. 



