160 ORIGIN OF SPECIES 



comes, through changed habits, superfluous, without by any 

 means causing some other part to be largely developed in a 

 corresponding degree. And, conversely, that natural selec- 

 tion may perfectly well succeed in largely developing an 

 organ without requiring as a necessary compensation the 

 reduction of some adjoining part. 



MULTIPLE,, RUDIMENTARY, AND LOWLY ORGANISED STRUC- 

 TURES ARE VARIABLE 



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

 Hilaire, both with varieties and species, that when any part 

 or organ is repeated many times in the same individual (as 

 the vertebrae in snakes, and the stamens in polyandrous flow- 

 ers) the number is variable ; whereas the same part or organ, 

 when it occurs in lesser numbers, is constant. The same 

 author as well as some botanists have further remarked that 

 multiple parts are extremely liable to vary in structure. As 

 "vegetative repetition," to use Prof. Owen's expression, is a 

 sign of low organisation, the foregoing statements accord 

 with the common opinion of naturalists, that beings which 

 stand low in the scale of nature are more variable than those 

 which are higher. I presume that lowness here 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 not have preserved or rejected each little deviation 

 of form so carefully as when the part has to serve for some 

 one special purpose. 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 purpose must be of some 

 particular shape. Natural selection, it should never be for- 

 gotten, can act solely through and for the advantage of each 

 being. 



Rudimentary parts, as it is generally admitted, are apt to be 

 highly variable. We shall have to recur to this subject; and 

 I will here only add that their variability seems to result from 

 their uselessness, and consequently from natural selection 

 having had no power to check deviations in their structure. 



