CONDITIONS OF LIFE 235 



well be tempted to enter the caves themselves, drawn thither 

 by the abundance of food, and the stress of competition in the 

 outside world. It is well known that fishes living in holes in 

 river banks or under stones tend to have reduced eyes, and it 

 is possible that the ancestors of the existing BHnd-fishes spent 

 their lives in such a manner. 



The peculiar Blind Goby {Typhlogobius) is found on the reefs 

 of the shores of southern California, fastened to the underside 

 of rocks or crawling about in the crevices or in the burrows 

 made by crustaceans. It is about two inches in length, pale 

 pink in colour, with a smooth, naked skin. The eyes are small 

 but functional in the young, but are mere vestiges hidden under 

 the skin in the adult fish. The head is well supplied with dermal 

 sensory organs (Fig. 92B) . The channels excavated by a species 

 of burrowing shrimp are used by at least two other kinds of 

 Goby as well as by the blind form, but these normally live outside 

 the holes and only retire into them when danger threatens, 

 whereas the Blind Goby never leaves the shelter of the burrows. 

 Further, the normal Gobies are found all over the shore in this 

 region, but the blind form is restricted in its distribution to a 

 particular part. There can be little doubt that the Blind Goby 

 is descended from a species which habitually sought the crevices 

 and holes in the rocks, and in which the eyes were already 

 reduced and the sensory papillae more extensively developed 

 than in most members of this tribe. 



Certain other Gobies (Evermannichthys, etc.) habitually live 

 inside sponges, the bodies of the little fishes being of an even 

 diameter which allows them to slip in and out of the larger 

 orifices of the sponge's surface. The scales of these forms are 

 mostly either absent or very feebly developed, but along the 

 lower posterior line of the sides there are two series of large, 

 well-separated scales, the edges of which are produced into 

 long spines, while a series of four more is situated in the middle 

 line behind the anal fin. It is suggested that these specialised 

 structures are used for climbing up the inner surfaces of the 

 sponge cavities. 



Another unusual environment to which many fishes have 

 successfully adapted themselves is provided by the torrential 

 streams of hills and mountain ranges. Here again the fishes 

 probably colonised the streams very gradually, being dri\en 

 from the more sluggish waters of the lowlands by the need for 

 further food supplies and the prevalence of enemies of all 

 kinds. Among the more interesting of the hill-stream forms 



