36 THE PRINCIPLES OF HEREDITY 



acquirements that are commonly useful to the species the 

 different species differ, as regards size and shape, and capacity 

 for action, not only in characters which are inborn, but also 

 in those which are acquired. Thus an ox differs in size and 

 shape from a man not alone in inborn characters, but also in 

 the characters which are acquired as a result of exercise and 

 use. The structures of both ox and man develop in response 

 to use, but not quite in the same direction, nor in the same 

 proportion, nor to the same degree ; hence to some extent 

 the differences in size and shape betwixt the two animals. 

 For instance, the fore limbs of both the ox and man grow 

 greatly in response to use, but the lines of growth are very 

 different. 



59. Adaptive acquirements may involve not only quantita- 

 tive changes, but qualitative changes also. For example, in 

 man exercise does not merely cause a muscle to increase in 

 size; it occasions besides, as in athletes after training, an 

 increase in efficiency (i. e. in the power and duration of con- 

 traction) which is greatly out of proportion to the increase of 

 size. Intermittent friction, or heat, or other irritant (e. g. 

 chemical) not merely causes the skin to thicken, as in corns 

 and callosities ; it renders it denser also. Again, stimulation 

 (that is, use) may result in change that is apparently wholly 

 qualitative. Thus eyes, which, when unaccustomed to the 

 task, are rendered sore by the continual scrutiny of small 

 obiects (e. g. print in the case of an adult learner) may by 

 practice be trained, without apparent physical change, to 

 endure the labour without damage. 



60. Since the power of profiting physically by experience, 

 of developing individually in response to use, is an enormously 

 important faculty which is observable mainly in the higher 

 animals, it is, speaking comparatively, quite a recent product 

 of evolution. So much is certain. 



61. Now we may summarize our facts, and see how they 

 bear on the problem of the transmission of use-acquirements. 

 Plants and plant-like animals if they " use " their parts do 

 so only to a small extent ; and therefore amongst them there 

 can be little or no " use-inheritance." Higher in the scale 

 (e. g. most insects and the lower vertebrates) animals use 

 their parts, but still make few use-acquirements, therefore 

 few use-acquirements can be transmitted. Yet higher in the 

 scale animals use their parts and make use-acquirements, 

 but, since every animal has to make the whole of these 

 acquirements afresh, none of them are transmitted. Were 

 use-acquirements transmitted, a child, for example, would 



