STRUCTURES VARIABLE. 133 



same author, as well as some botanists, have further re- 

 marked 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 fore- 

 going statements accord with the common opinion of natur- 

 alists, 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 care- 

 fully 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 conviction 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 



