Chap. V. CORRELATION OF GROAVTH. 149 



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

 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 ^art 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 struc- 

 ture. Inasmuch as this " vegetative repetition," to use 

 Prof. Owen's expression, seems to be a sign of low organi- 

 sation ; 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. 



Eudimentary 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 having 

 no power to check deviations in their structure. Thus 



