588 ON ABNORMAL REPETITION OF PARTS IN ANIMALS. [Juiie 17, 



that those of the Herring bear concentric lines which are almost 

 always smooth and without serrations, while those of the Pilchard 

 are marked with lines which are waved into very characteristic 

 crenelated serrations. On comparing the scales which are repeated, 

 it was found that they also show these characteristic serrations 

 and that in pattern they differ in nowise from the scales of the 

 Pilchard. This evidence appears to tell very strongly against 

 the theory that the small scales are derived from a Herring 

 parent. 



The evidence from the gill-rakers seems to be also unreliable. In 

 a normal Pilchard Mr. Day found 71 on the hypo- and cerato-bran- 

 chials of the first gill-bar, and in a specimen examined by me 72 

 were present and in normal Herrings 48. But in the two specimens 

 showing the repeated scales there were present, on the normal sides 

 79 and 67 respectively, and on the abnormal sides 78 in the one 

 fish and 67 in the other. In size and shape the gill-rakers were 

 like those of the Pilchard, being smooth, and unlike those of the 

 Herring, which bear well-marked teeth. 



As it is stated that the serrations characteristic of the operculum 

 of the Pilchard were very distinct on the abnormal side, it is im- 

 possible to place much stress on the circumstance that they were 

 less distinct than those of the other side. 



In addition to the considerations given above^ there are several 

 a priori objections to the hypothesis of the hybrid origin of these 

 forms ; as, for example, that unilateral division of parental characters 

 is certainly not a common phenomenon, if it occurs at all, and so on. 

 But since the evidence advanced for the theory of hybrid parentage 

 is already open to criticism, it is perhaps unnecessary to discuss these 

 further difficulties. 



On the whole, therefore, it seems simpler to look on these abnor- 

 malities as instances of the phenomenon of Repetition of Parts, 

 which is so common a form of variation. Though on the present 

 occasion a discussion of the nature of these variations is to be avoided, 

 it may be useful to mention in this connexion that such repetitions 

 are especially common among exoskeletal structures ; and though, 

 in the absence of fuller treatment, the comparison may seem some- 

 what crude, reference may be permitted to such cases as that of the 

 Merino Sheep &-c., in which the number of hair-follicles in a given 

 area is enormously greater than that in the common varieties. Such 

 variations are well known among many wild and domesticated 

 animals. The unilateral occurrence of siich a variation, however, is 

 exceptional. 



The fact that these fishes were full-grown and in good condition, 

 swimming with the shoal, should be specially remarked. 



For the reasons given above it is felt to be unadvisable to consider 

 the significance of these facts until it shall be possible to discuss the 

 whole question of the Variation of Multiple Parts. 



