156 OUTLINES OF EVOLUTIONARY BIOLOGY 



edge. This may be compared to the process of mutation or discon- 

 tinuous variation, whereby an organism acquires a new type of 

 structure. 



Concerning the forces which bring about mutation in organisms 

 we know little or nothing. It is possible that in some cases the 

 mutation may be gradually prepared within the germ cells long 

 before it manifests itself the final upsetting of the equilibrium 

 only taking place on the addition of the last straw. If such be 

 the case there is no need to suppose that mutations differ 

 essentially in nature from small, fluctuating variations. Tower's 

 observations on the artificial production of mutations in certain 

 beetles, however, indicate very clearly that such modifications 

 may apparently arise quite suddenly as the result of some change 

 in the environment acting directly upon the germ cells of the 

 parent. These observations will be referred to again at the close 

 of the present chapter. 



Somatoyenic Variations. Characters are said to be somatogenic 

 or " acquired " when they arise in the life-time of the individual 

 exhibiting them and owe their origin to the direct influence of 

 the environment upon the soma or body. They are, usually at 

 any rate, not transmitted by heredity to succeeding generations, 

 except perhaps to a very limited and inappreciable extent. 1 

 Under this heading are included the effects of use and disuse 

 of organs, and numerous cases in which modifications of the 

 body are artificially produced, as well as those in which they are 

 due to natural causes. 



Amongst the effects of use and disuse we may mention, on the 

 one hand, the enlargement or the atrophy of special organs con- 

 sequent upon the extent to which they are employed, and, on the 

 other, the effects of education. The muscles of an athlete may 

 be greatly increased in size by constant use, and similarly if one 

 of the two kidneys be removed the other, having more work 

 thrown upon it, becomes enormously enlarged ; but we should not 

 expect these modifications to be handed on to the next generation. 

 Children have been taught to speak ever since man first became 

 differentiated from his speechless ancestors, but every child has to 

 learn the art anew and if brought up amongst foreigners will 

 come to speak a language different from that of its parents. 



The small feet of Chinese ladies and the slender waists of 

 many Europeans are artificially produced somatic modifications 



l This may be, however, a very important exception. 



