8TRUCTURE8 VARIABLE. 141 



the vertebrae in snakes, and the stamens in pol3'androu8 

 flowers) 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 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 natu- 

 ralists, 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 re- 

 sult from their uselessness, and consequently from natural 

 selection having had no power to check deviations in their 

 structure. 



A PART DEVELOPED 11^ ANY SPECIES IN AN EXTRAORDI- 

 NARY 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 bo 

 here introduced. I can only state my conviction that it is 

 a rule of high generality. I am aware of several causes of 



