250 THE PRINCIPLES OF HEREDITY 



by a concurrent regression of the power of developing in 

 response to other stimuli nutrition, heat, moisture, and 

 even injury. Thus the human being, who of all animals has 

 the greatest capacity for making use-acquirements, has the 

 least power of reaching full development of body and mind 

 without the aid of use and experience. In the higher 

 animals, therefore, use has replaced, to a great extent, other 

 stimuli (except conjugation) as an incentive to development. 

 Probably this is true as regards the invertebrates also. 1 



411. The acquirements which result from the stimulus of 

 use are mere extensions of those which result from nutrition 

 and the other stimuli which early affect the germ and the 

 tissues that develop from it. In other words, the acquirements 

 which result from use are mere extensions of those which are 

 technically termed " inborn " characters. Natural Selection 

 has so dealt with germs and the tissues which arise from 

 them that they respond to given stimuli by developing in 

 definite ways. At first in all animals, in response to nutrition, 

 are developed structures capable of being used. Later in 

 some animals the structures develop farther in response to 

 being used. In other words, at given stages of the development 

 of the higher animals, use takes up the task for which 

 nutrition alone has become inadequate. Use plays a great 

 part in the development of man, and, therefore, very much of 

 the subsequent development of the helpless baby results from 

 it ; it plays a lesser part in the development of the horse, and, 

 therefore, comparatively little of the more active foal's 

 development depends on it, it hardly plays any part in the 

 development of the frog, and probably none at all in that of 

 the dragon-fly. 



412. Since acquirements are mere extensions of inborn 

 characters (so-called) we are often unable to distinguish 

 between the two to say where the inborn character ends 



that caused them to grow to a closer adaptation, to the changes in the 

 environment than was possible had they developed solely under the 

 blind influence of nutrition and the other stimuli which develop the 

 ' ' inb or n " ch aracters. 



1 Scratching an itching spot furnishes an instance of an acquirement 

 replacing an instinct. Normally we itch when some small object rests 

 on or moves over the skin, and especially when we get a minute bite. 

 Scratching gives relief, and is exactly adapted to dig out or tear away 

 the object. Probably it was even more useful to our remote ancestors 

 than to us. The action is certainly instinctive in some if not all birds, 

 and in the lower mammals. Equally certainly it is not instinctive in 

 man, for the baby does not scratch itself, till it has learned how, and 

 when, and where to scratch. 



