308 THE INDUCTIONS OF BIOLOGY. 



or climate to another, frequently undergoes what botanists 

 call " change of habit ^' — a change which, without affecting 

 its specific characters, is yet conspicuous. In its new locality 

 the species is distinguished by leaves that are much larger 

 or much smaller, or differently shaped, or more fleshy; or 

 instead of being as before comparatively smooth, it becomes 

 hairy ; or its stem becomes woody instead of being herbaceous ; 

 or its branches, no longer growing upwards, assume a droop- 

 ing character. Now these " changes of habit " are clearly 

 determined by functional changes. Occurring, as they do, in 

 many individuals which have undergone the same transporta- 

 tion, they cannot be classed as " spontaneous variations." 

 They are modifications of structure consequent on modifica- 

 tions of function that have been produced by modifications 

 in the actions of external forces. And as these modifications 

 re-appear in succeeding generations, we have, in them, ex- 

 amples of functionally-established variations that are here- 

 ditarily transmitted. 



Evidence of analogous changes in animals is difficult to 

 disentangle. Only among domesticated kinds have we any 

 opportunity of tracing the results of altered habits ; and here, 

 in nearly all cases, artificial selection has obscured them. 

 Still, there are some facts which seem to the point. Mr. 

 Darwin, while ascribing almost wholly to " natural selection '^ 

 the production of those modifications which eventuate in 

 differences of species, nevertheless admits the effects of use 

 and disuse. He says — " I find in the domestic duck that the 

 bones of the wing weigh less and the bones of the leg more, 

 in proportion to the whole skeleton, than do the same bones 

 in the wild duck; and I presume that this change may be 

 safely attributed to the domestic duck flying much less, and 

 walking more, than its wild parent. The great and inherited 

 development of the udders in cows and goats in countries 

 where they are habitually milked, in comparison with the 

 state of these organs in other countries, is another instance of 

 the effect of use. Not a single domestic animal can be named 



