Chap. V. MULTIPLE STKUCTUKES VAKIABLE. 1.51 



ly, that natural selection may perfectly well succeed in largely 

 developing any organ, without requiring as a necessary com- 

 pensation the reduction of some adjoinhig part. 



Mullij)le, liudimentaryy and Loichj-organized Structures are 



variable. 



It seems to be a rule, as remarked by Is. Geoffroy St.-IIi- 

 laire, both in varieties and in species, that, when any part or 

 organ is repeated many times in the structure of the same 

 individual (as the vertebnu in snakes, and the stamens in poly- 

 androus ilowers), tlie number is variable; whereas the number 

 of the same part or organ, when it occurs in lesser num])ers, 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 

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

 tion, the foregoing remark seems connected with the very gen- 

 eral opinion of Jiaturalists that beings low in the scale of Na- 

 ture arc more variable than those which are liigher. I pre- 

 sume that lowncss in this case means that the several parts of 

 the organization have been but little specialized for particular 

 functions ; and, as long as the same part has to perform diver- 

 sified Avork, wo can perhaps see Avhy it should remain variable, 

 that is, why natural selection should not have preserved or 

 rejected eacli little deviation of form so carefully as when the 

 part had 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, while a tool for some particular j^urpose 

 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. 



Kudimentary parts, it has been stated by some authors, and 

 I believe with truth, are apt to be highly varialjle. AVe shall 

 have to recur to tlie general subject of rudimentary and aborted 

 organs; and I will here only add tliat tlieir variability seems 

 to be owing to their uselessness, and therefore to natiu-al selec- 

 tion having no power to check deviations in their structure. 

 Thus rudimentary parts are left to the free play of the various 

 laws of growth, to the effects of long-continued disuse, and to 

 tlic tendency to reversion. 



