2i8 A STUDY IN HEREDITY 



attributed to the accumulation, during generations, of inborn varia- 

 tions alone, but must, in part, be attributed to the accumulation, 

 during generations, of the effects of use and disuse, i.e. to the 

 accumulation of acquired modifications. 



But modifications acquired as a result of use and disuse, are 

 plainly never transmitted. Thus, an infant's limb never attains to 

 the adult standard except in response to the same stimulation 

 (exercise) as that which developed the parent's limb. The same 

 is true of all the other structures which, in the parent, underwent 

 development as a result of use, or subsequent retrogression in the 

 absence of it. These, like the limbs, do not develop or retro- 

 gress in the infant, except as a result of similar causes. Plainly, 

 then, what is transmitted to the infant is not the modification, 

 but only the power of acquiring it under similar circumstances — 

 a power which has undergone such an evolution in high animal 

 organisms that, as I say, in man, for instance, all the develop- 

 mental changes which occur between infancy and manhood are 

 attributable to it. It follows, therefore, that the exquisite co- 

 ordination of all the parts of a high animal is not due to the 

 inherited effects of use and disuse, but to this great power of 

 acquiring modifications along certain definite lines ; so that if an 

 animal varies in such a way as to have one of its structures (e.g. 

 horn, which is wholly inborn) larger than the parent had, then all 

 the other structures associated with it, owing to the increased 

 strain (i.e. the increased stimulation) put on them, undergo a 

 corresponding modification, and thus preserve the harmony of all 

 the parts of the whole. So also, if the horn, for instance, be 

 smaller than in the parent, the lesser strain placed by it on the 

 associated structures causes these also to develop less than in 

 the parent, whereby, again, the harmony of the whole is 

 preserved. 



I have dwelt at greater length on this neglected subject of 

 acquired characters (properly so-called) elsewhere, but I think 

 I have said enough even here to demonstrate its immense 

 importance. The power of acquiring fit modifications in 

 response to appropriate stimulation is that which especially 



