LAWS OF VARIATION 135 



fluous, without by any means causing some other part 

 to be largely developed in a corresponding degree. 

 And, conversely, that natural selection may perfectly 

 well succeed in largely developing any organ, without 

 requiring as a necessary compensation the reduction of 

 some adjoining part. 



It seems to be a rule, as remarked by Is. GeofFroy 

 St. Hilaire, both in varieties and in species, that when 

 any part or organ is repeated many times in the struc- 

 ture of the same individual (as the vertebrae in snakes, 

 and the stamens in polyandrous flowers) the number is 

 variable ; whereas the number of the same part or 

 organ, when it occurs in lesser numbers, is constant. 

 The same author and some botanists have further 

 remarked that multiple parts are also very liable to 

 variation in structure. Inasmuch as this ' vegetative 

 repetition,' to use Professor Owen's expression, seems to 

 be a sign of low organisation, the foregoing remark seems 

 connected with the very general opinion of naturalists, 

 that beings low in the scale of nature are more variable 

 than those which are higher. I presume that lowness 

 in this case means that the several parts of the 

 organisation have been but little specialised for 

 particular functions ; and as long as the same part has 

 to perform diversified work, we can perhaps see why it 

 should remain variable, that is, why natural selection 

 should have preserved or rejected each little deviation 

 of form less carefully than when the part has to serve 

 for one special purpose alone. In the same way that 

 a knife which has to cut all sorts of things may be of 

 almost any shape ; whilst a tool for some particular 

 object had better be of some particular shape. Natural 

 selection, it should never be forgotten, can act on each 

 part of each being, solely through and for its advantage. 



Rudimentary parts, it has been stated by some 

 authors, and I believe with truth, are apt to be highly 

 variable. We shall have to recur to the general 

 subject of rudimentary and aborted organs ; and I will 

 here only add that their variability seems to be owing 

 to their uselessness, and therefore to natural selection 



