EYE OF NOTORTCTES TYPHLOPS. 567 



experiment is one of evolution without the assistance or 

 intervention of natural selection." This latter statement 

 may or may not be true in tbe case of the blind fishes with 

 which he is dealing. To explain their degeneration he 

 invokes the aid solely of " disuse " of the eyes in " animals 

 already predisposed to shun the light, or creep under rocks 

 or into crevices" [4, p. 535]. 



But there can be no question of use or want of use in the 

 eyes of Notoryctes, which " is, in reality, a more surface 

 animal than the European mole/' as already pointed out by 

 Professor Spencer in the report of the Horn Expedition [9, 

 p. 51]. 



Here several factors have come into play. 



First, as the result of natural selection, degeneration has 

 taken place because the presence on the surface of the head 

 of such a sensitive structure would be deleterious to the 

 animal. "The fine grains of sand through which it burrows 

 Avould have been a fruitful source of irritation, resulting 

 constantly in the production of inflammation," and, as Pro- 

 fessor Spencer continues, " more than counter-balancing the 

 advantage to be gained from the possession of an eye when 

 it did come to the surface" [9, p. 51]. 



Second, when the eye had receded beyond the reach of 

 irritation this factor would no longer operate. Then the fact 

 of disuse may have operated to intensify the degeneration 

 already well advanced, at all events in the accessory parts, 

 such as the eyelids and character of the muscles. 



Thirdly, the great degeneration of the eyeball itself with 

 its nervous structures has undoubtedly gone on side by side 

 with the great development of the gland structures both in 

 the eye region and in the nasal region [see 12] itself an 

 adaptation to the burrowing habit, and so probably controlled 

 by direct natural selection. Possibly the used organ has 

 developed at the expense of the food and room of the disused 

 organ by the law of compensation and economy of growth. 

 It cannot be due to merely disuse or diminished use from 

 want of light, since disused organs that are not concerned in 



