The second type of variability manifested by instinct is its capacity for complex and continuous adjustment to variable environmental circumstances. Thus predatory animals, such as wasps, crabs, spiders, and carnivorous mammals, accommodate themselves appropriately and uninterruptedly to the changing and unforeseeable movements of the prey they are engaged in stalking, giving evidence in this way of the regulation of their hunting instincts by sensory impressions. Whether this element of psychic control is based upon object-perception, or simple sensation, and whether it involves a sensual impulse, or is merely sensori-motor, we have, naturally, no direct means of ascertaining. But the presence of some sort of sensory regulation is evident enough, _e.g._ in the prompt and unerring flight of vultures to distant carrion. Moreover, there is a close analogy between our sense organs and those of an animal. Particularly, in the case of the higher animals, the resemblance of the sense organs and nervous system to our own is extremely close, so much so that even the localization of sensory and motor centers in the brain is practically identical in dogs, apes, and men. Moreover, the animals make analogous use of their sense organs, orientating them and accommodating them for perception, and using them to inspect strange objects, etc., _e.g._ they turn their eyes, prick up their ears, snuff the wind, etc. Again, analogous motor and emotional effects result from the stimulation of their sense organs, and brutes make emotional displays of anger, exultation, fear, etc., similar to our own. Hence it is to be presumed that they have similar sensuous experiences. The analogy, however, must not be pressed further than the external manifestations warrant. With brute animals, the manifestations in question are confined exclusively to phenomena of the sensuous order.