LAWS OF VARIATION 121 



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 "vegetable repetition," to use Professor 

 Owen's expression, is a sign of low organization, 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 organization have been but little 

 specialized 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; while a tool for some particular purpose 

 must be of some particular shape. Natural selection, it should 

 never be forgotten, can act solely through and for the advantage 

 of each being. 



Rudimentary parts, as 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. 



A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE 

 OR MANNER, IN COMPARISON WITH THE SAME PART IN ALLIED 

 SPECIES, TENDS TO BE HIGHLY VARIABLE 



Several years ago I was much struck by a remark to the above 

 effect made by Mr. Waterhouse. Professor Owen, also, seems to 

 have come to a nearly similar conclusion. It is hopeless to attempt 

 to convince any one of the truth of the above proposition without 

 giving the long array of facts which I have collected, and which 

 cannot possibly be here introduced. I can only state my convic- 

 tion that it is a rule of high generality. I am aware of several 

 causes of error, but I hope that I have made due allowances for 

 them. It should be understood that the rule by no means applies 

 to any part, however unusually developed, unless it be unusually 

 developed in one species or in a few species in comparison with 

 the same part in many closely allied species. Thus, the wing of 

 the bat is a most abnormal structure in the class of mammals, but 

 the rule would not apply here, because the whole group of bats 

 possesses wings; it would apply only if some one species had 

 wings developed in a remarkable manner in comparison with the 



