sect, v.] INTRODUCTION. 25 



sequence of this that the subject of Symmetry and Repetition of 

 Parts has a second and indirect bearing on the Study of Variation 

 which is scarcely less important than the direct bearing of which 

 mention has been made above. 



This indirect bearing on the manner of origin of Specific 

 Differences arises from a circumstance which in treatises on 

 Evolution is commonly overlooked. In comparing a species in 

 which parts are repeated, with an allied species in which the same 

 parts are repeated, it commonly occurs that each of the repeated 

 parts of the one have some character by which they are dis- 

 tinguished from the like parts of the other. This differentiating 

 character may be a qualitative one, or a numerical one, or both. 

 In such cases it very frequently happens that this character occurs 

 in each member of the series of Repetitions. For example, the 

 tarsi of the Weevils have only four visible joints, while those 

 of the majority of beetles have five ; but the characteristic 

 division into four joints occurs in each of the legs. Before the 

 four-jointed character as seen in the Weevils could be produced 

 it was necessary that not one but all of the legs should vary from 

 the five-jointed form, and in this particular way. The leaves on a 

 beech tree are all beech leaves, and if the tree is a fern-leaved 

 beech, they may, and generally speaking do, all shew the charac- 

 ters of that variety ; and so on with other particular species and 

 varieties. 



The limbs of a bilaterally symmetrical animal, in which the 

 right side is the image of the left, are of course alike, and any 

 specific character which is present in the limbs of the one side 

 must in such an animal be normally present in those of the other 

 side. 



The same is true of many forms in which appendages are 

 repeated in series, as for example, the fore-legs and hind-legs 

 of the Horse, the fore- and hind-wings of the Brimstone Butterfly 

 {Gonepteryx rhamni); of the patterns on several segments of many 

 caterpillars ; of the patterns of the segmental setae of many worms, 

 and so forth. In series whose members are differentiated from each 

 other, it of course frequently happens that the same specific 

 characters are not present in all the members of the series, and in 

 nearly all such, cases these characters are not presented by all in 

 equal degree ;<nevertheless substantially the phenomenon remains 

 that similar characters often are presented by the several members 

 of a series of repeated organs, - 



To many this will seem little better than a truism, neverthe- 

 less I offer no apology for its introduction ; for though, as a 

 common and obvious fact, it is a truism, it is besides a truth, the 

 far-reaching significance of which is scarcely appreciated. For, in 

 the consideration of the magnitude of the integral steps by which 

 Variation proceeds, we shall have this to remember : that to 

 produce any of the forms of which we have spoken, by Variation 



