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. 
