CHAP, xviii.] BILATERAL REPETITION. 449 



there is thus a clear proof that so far as the variations in number and 

 symmetry are concerned, the transition from the one form to the other 

 may be discontinuous. 



Analogous phenomena in animals are so familiar that general 

 description of them is for the most part not needed, and an 

 account will only be given of a few less known examples both 

 of union and of division of such parts. Besides these strictly 

 Meristic Variations in the amount of separation between the two 

 halves a few examples are introduced in further illustration of 

 the relationship that subsists between the two halves of a bilateral 

 animal. 



In considering the evidence both of median union and of 

 division it must be remembered that the germs of most of the 

 organs in question are at some time of their developmental history 

 visibly double, and that when organs that should normally unite 

 to form single median structures are found double in older stages, 

 this duplicity is strictly speaking only a persistence of the earlier 

 condition. But to appreciate this comment it should be extended. 

 For, in every animal in which at some period of the segmentation 

 of the ovum, the plane of one of the cleavages corresponds with 

 the future middle line, all median organs must in a sense be paired 

 in origin, and the distinction between paired and median organs 

 is thus seen to be only one of the degree or amount of separation 

 between the symmetrical halves. Nevertheless the evidence of 

 Variation bears out the expectation that would be formed on 

 examination of normal diversities between species or larger groups 

 both in animals and plants, namely that whenever structures are 

 geometrically related to each other as optical images, insta- 

 bility may shew itself as Variation in the degree to which such 

 parts unite with or separate from each other. It is remarkable 

 that this instability appears as much in the case of organs bi- 

 laterally symmetrical about an axis of Minor Symmetry as it does 

 in the parts paired about the chief axis of Symmetry of the 

 whole body. 



Examples of such Variation in bilaterally symmetrical parts 

 of a Minor Symmetry have been already given in the case of 

 the feet of the Horse and of the converse phenomenon in the 

 feet of Artiodactyles (q.v.). 



A good illustration of the way in which duplicity about an 

 axis of Minor Symmetry may pass into the unpaired condition 

 is seen in the case of ocellar markings on bilaterally symmetrical 

 feathers. By comparing different feathers on several species of 

 Polyplectron, DARWIN found that it was possible to find most of 

 the gradations between the complete duplicity shewn in Fig. 140, 

 I. where each half of the feather bears an almost symmetrical 



# / 



ocellus, and the partially confluent condition shewn in Fig. 140, II, 

 which is not far removed from the state of the ocellus in the 

 Peacock's tail-coverts, where the whole ocellus has no peripheial 



B. 29 



