Evolution of Animals 461 



morphological condition that has been variously interpreted. 

 Owing to the burrowing habit of many Apoda, and the burrow- 

 ing or cave-dwelling habit of some Urodela, a certain degenera- 

 tion has undoubtedly taken place, usually in the eyes, to some 

 extent in the color and consistency of the skin. There seems 

 however small evidence that the degeneration has become 

 fundamental or deep-seated. But the often tacit, though in 

 some cases expressed, opinion has gained ground that these 

 groups are decidedly degraded, rather than types that link 

 up for us an evolving line. We can now examine the e\ddence. 

 But before doing so we would express the opinion that the 

 very burrowing or retreating habits of these animals are prob- 

 ably the factors that have caused them to be left as connecting 

 types of high interest at the present day. 



The general primitive character of the endoskeleton and 

 of other parts in cyclostomes agrees well witTi the idea that 

 these have not yet evolved limb-girdles or limbs. The active 

 gliding habits and slippery skin, also, scarcely serve to set 

 up the needed irritable stimuli that would start paired limbs 

 as a response result. The constant balancing efforts amid 

 marsh vegetation, or progression on land, have called forth and 

 established this by degrees, as we shall trace soon. 



The presence in carboniferous strata of early apodous ba- 

 trachians, that resembled in many points those of recent date, 

 at once proves that the type is a very ancient one, and that 

 an ample period has elapsed for the evolution of more complex 

 forms from them, at the same time that some, from habit 

 and environment, have retained primitive conditions largely 

 unchanged up to the present day. We would therefore, and 

 for many other reasons, regard the existing apodous condition 

 as truly ancestral, and as being retained owing to continued 

 preservation of a hke balance between structure and environal 

 relation. But the finding by Brauer (96: 500) of "a pair of 

 little swellings behind the last gill cleft, and an unpaired swell- 

 ing (corresponding with a double one in Ichthyophis) in front 

 of the vent," is of great interest. For "not unreasonably he 

 sees in these swellings the last, very transitional vestiges of 



