| OCR Text |
Show 588 ON ABNORMAL REPETITION OF PARTS IN ANIMALS. [June 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 aud that in pattern they differ in nowise from ihe 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 iri 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 impossible 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 abnormalities 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 somewhat 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 such 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. |
