THE ANOESO^BY OF THE OHORDATA. 77 



number of these is liable to great variations, not even being 

 constant in the species. For example, certain deer and also 

 certain sheep have specifically more horns than two ; and in 

 the case of Iceland sheep the horns may be three, four, or five 

 (Youatt, 'The Sheep'). By the nature of the case none of 

 these repetitions can be atavistic ; and it is interesting to notice 

 how, just as it was shown that irregular repetitions of parts 

 about the axes of symmetry of the body often take up regular 

 secondary relations to them, recurring either in segmental 

 pairs or in radial symmetry, so these minor repetitions take up 

 regular relations (secondary in some cases, probably primitive 

 in others) to the axes of the limb or part of the body in which 

 they occur. Thus the ossifications in the Crinoid stem or 

 the Starfish arm are so regularly related to the axis of the 

 part that in the latter case they have suggested to Haeckel his 

 extraordinary view of the phylogeny of the group, appearing to 

 him precisely similar to the segmentation of a Chsetopod, The 

 case of the scales of fishes and the hairs and markings of cater- 

 pillars should perhaps have been more properly quoted in the 

 former connection, as being an instance of irregular repetitions 

 which have become definitely related to the symmetry, as in 

 the case of the Sturgeon, and among caterpillars the Tussocks 

 and the Spherigidse. One very curious instance may be 

 quoted of a series of repetitions which, though essentially 

 arranged with reference to the axis of a limb, have yet a defi- 

 nite relation to the long axis of the body. This instance is 

 that of the Vertebrate tail, which has often been adduced by 

 opponents of the Annelid theory of Vertebrate descent. Now, 

 the structures which repeat themselves in the Vertebrate tail 

 with great variability of number, namely, the vertebrse with 

 their neural and haemal arches, the segmental vessels and 

 nerves, &c., are precisely those structures upon whose repeti- 

 tion in the trunk the view of the primitive character of the 

 segmentation of the Vertebrata mainly depends. 



In the foregoing pages the attempt has been made to show 

 that greater or less repetition of various structures is one of 

 the chief factors in the composition of animal forms, that these 



