Chap. V. 
COKEELATION OF GEOWTH. 
149 
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 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 
