62 Resemblances and Di^erences Among Living Things 



comparatively slight disturbance from outside. In photogra- 

 phy we use a great many substances that undergo chemical 

 change under exposure to light. In these cases, the response 

 to the disturbance results in the destruction of the sensitive 

 substance. In an organism, however, the response to a dis- 

 turbance is, generally speaking, of a kind that tends to help 

 the organism in some way, or of a kind that tends to preserve 

 it from further injury. 



When a moving body approaches a bird, the bird flies 

 away. That is, the stimulation (in this case through the 

 eye) brings about a response which in general tends to save 

 the animal from possible injury. On the other hand, if the 

 bird catches sight of a very small moving object, such as an 

 insect or worm, the movements of the bird's muscles are 

 such as tend to capture the small body as possible food. When 

 a dog is annoyed, he snaps in the direction from which the 

 disturbance comes. When a hedgehog is disturbed, it bristles 

 out in the direction from which the irritation comes — 

 although it does not, as a common superstition tells us, throw 

 out its quills towards the possible enemy. From the simplest 

 animals like the ameba to the most complex the general fact 

 holds that response to stimulus is on the whole, though not 

 uniformly, of a kind that tends to protect, or to obtain 

 something necessary to continue life. Adaptation, while 

 never one hundred per cent perfect, is characteristic of all 

 living bodies. These two general facts, namely, adaptation 

 and imperfection of adaptation, are very important for un- 

 derstanding of life, and have a particular significance for 

 the question of evolution. 



Origin 



Man with his ingenuity has succeeded in building ma- 

 chines that show remarkable adaptation of parts to the 

 maker's purpose, remarkable complexity of structure in 

 which each detail fits in with every other detail. He has 

 constructed mechanisms that are remarkably sensitive to 



