SECT, xiv.] INTRODUCTION. 79 



there is a progression or increase of size and degree of development, 

 starting from a small terminal member. In such cases, as that of 

 the last rib in Man, and several other animals, the wisdom-teeth of 

 Man, etc., it is quite true that in the terminal member Variation is 

 more noticeable than it is in the other members. This is, I 

 believe, a consequence of the mechanics of Division, and has no 

 connexion with the fact that the functions of such terminal parts 

 are often trifling. Upon this subject something will be said later 

 on, but perhaps a rough illustration may make the meaning more 

 clear at this stage. If a spindle-shaped loaf of bread, such as a 

 "twist," be divided with three cuts taken at equal distances, in 

 such a way that the two end pieces are much shorter than the 

 middle ones, to a child who gets one of the two large middle 

 pieces the contour-curves of the loaf will not matter so much ; but 

 to a child who gets one of the small end bits, a very slight altera- 

 tion in the curves of the loaf will make the difference between a 

 fair-sized bit and almost nothing, a difference which the child will 

 perceive much more readily than the complementary difference in 

 the large pieces will be seen by the others. An error in some 

 measure comparable with this is probably at the bottom of the 

 statement that useless parts are variable, but of course there are 

 many examples, as the pinna of the human ear, which are of a 

 different nature. It is unnecessary to say that for any such case 

 in which a part, apparently useless, is variable, another can be 

 produced in which some capital organ is also variable ; and 

 conversely, that for any case of a capital organ which is little 

 subject to Variation can be produced a case of an organ, which 

 though trifling and seemingly " useless," is equally constant. 

 With a knowledge of the facts of Variation, all these trite generali- 

 ties will be forgotten. 



5. Adaptation. 



In examining cases of Variation, I have not thought it neces- 

 sary to speculate on the usefulness or harmfulness of the variations 

 described. For reasons given in Section II, such speculation, 

 whether applied to normal structures, or to Variation, is barren and 

 profitless. If any one is curious on these questions of Adaptation, 

 he may easily thus exercise his imagination. In any case of 

 Variation there are a hundred ways in which it may be beneficial, 

 or detrimental. For instance, if the " hairy " variety of the moor- 

 hen became established on an island, as many strange varieties 

 have been, I do not doubt that ingenious persons would invite us 

 to see how the hairiness fitted the bird in some special way for life 

 in that island in particular. Their contention would be hard to 

 deny, for on this class of speculation the only limitations are those 

 of the ingenuity of the author. While the only test of utility is 

 the success of the organism, even this does not indicate the utility 



