246 THE INDUCTIONS OF BIOLOGY. 



they cannot be classed as " spontaneous variations. " They 

 are modifications of structure, consequent on modifications of 

 function, that have been produced by modifications in the 

 actions of external forces. And as these modifications re-ap- 

 pear in succeeding generations, we have, in them, examples 

 of functionally-established variations that are hereditarily 

 transmitted. Further evidence is supplied by what 



are called ' ' sports " in plants. These are of two kinds — the 

 gamogenetic and the agamogenetic. The gamogenetic may 

 be ascribed wholly to " spontaneous variations ;" or if they are 

 partly due to the inheritance of structural changes that are 

 produced by functional changes/ this cannot be proved. But 

 where the individuals displaying the variations arise by 

 agamogenesis, the reverse is the case : spontaneous variation 

 is out of the question ; and the only possible interpretation is 

 deviation of structure caused by deviation of function. A 

 new axis which buds out from a parent-axis, assumes an un- 

 like character — gives off lobed leaves in place of single leaves, 

 or has an otherwise different mode of growth. This change 

 of structure implies change in the developmental actions 

 which produced the new bud — change, that is, in the actions 

 going on in the parent shoot — functional change. And 

 since the modified structure thus impressed on the new shoot 

 by modified function, is transmitted by it to all the shoots 

 it bears ; we are obliged to regard the case as one of acquired 

 modification that has become hereditary. 



Evidence of analogous changes in animals, is difficult to 

 disentangle. Only among domesticated animals, have we any 

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

 in nearly all cases, artificial selection has obscured the results. 

 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 pro- 



