MERISTIC VARIATION 51 



that ever has four molars on both jaws, 1 which goes far to indi- 

 cate a marsupial ancestry. 



Remembering that the teeth are considered as one of the few 

 most reliable bases for classification, the remarkable variation in 

 their number, character, and position throws no little light on 

 the manner in which variation behaves, which is the chief reason 

 for their extended notice here. 



Supernumerary eyes. The development of extra eyes seems 

 to be confined to insects, which afford a number of excellent 

 examples of the development of normal tissue in abnormal 

 situations. 



Bateson's Nos. 419 to 421 are all cases of the development 

 of a third eye in Coleoptera. In every case these extra eyes are 

 quite distinct from the normal. In No. 419 the supernumerary 

 was small and lay abutting against but distinct from the right 

 eye. Its color was brownish yellow, while the normal eye was 

 black. In No. 420 the extra eye was on the left side but quite 

 independent of the normal eyes, which were exactly alike. In 

 No. 421 the extra eye was on the left side of the head, which 

 was rather less developed than the right. This eye is borne 

 upon an irregular chitinous loop, having a diameter of about 

 2.5 mm. This loop is attached to the substance of the head 

 before and behind, and these two attachments are distant from 

 each other about i mm. The diameter of the eye is about 2.5 mm., 

 thus occupying the full surface of the" loop, and its faceting is 

 said to be "not quite regular, and finer and slighter than that of 

 the normal eye." It is thus a very good attempt at a functional 

 third eye. 



Supernumerary wings in insects. Bateson 2 reports and de4 

 scribes fifteen cases of extra wings among insects, sometimes 

 on one side, sometimes on the other, but generally if not always 

 smaller than the normal ; sometimes plainly identified with the 

 fore wing, more frequently with the posterior ; occasionally nor- 

 mal in coloring and scaling, but as a rule abnormal. In one in- 

 stance it took the form of a large upright scale and in another of 

 a winglike appendage to the left anterior wing. 



1 Lydekker, Library of Natural History, p. 580. 



2 Bateson, Materials, etc., pp. 281-285. 



