604 AMPHIBIANS 



Comparison with Fishes — We should naturally like to find, in the 

 eyes of the chondrostean— >dipnoan-^crossopterygian series of fishes, 

 the prototypes of all the distinctively amphibian features. This is not 

 possible; and we may never have the story much more complete, even if 

 Latimeria is retaken and is studied by the right people. The eye of, say, 

 Amby stoma tigrinum compares quite strikingly with that of Protopterus; 

 but many of the similarities are matters of anatomy and of optics, and 

 our attention here should be strictly on the morphology. Again, it is 

 certain that the Protopterus eye has been secondarily simplified, and it 

 is very likely indeed that the most complex of modern urodele eyes lack 

 many features which the first urodeles possessed. Could we but restore 

 the lost details to both Protopterus and Ambystoma, we might still find 

 amazing similarity — or we might be unpleasantly surprised. When two 

 structures, complex in two different ways, are simplified secondarily they 

 may become closely identical without this having the least phylogenetic 

 significance. Witness the similarity of the eye of Protopterus to that of 

 a brook lamprey, no more closely related than an owl is to a gecko. 



Such things as the amphibian tadpole's spectacle, and its spherical 

 lens, are lungfish-like only because the tadpole is aquatic. The fact that 

 the Protopterus lens lies behind the iris means only that the eye is dis- 

 harmonious, not that it is pre-adapted for aerial vision. Some other parts 

 of the Protopterus eye — the chorioid, for example — are too much re- 

 duced to afford any comparisons. Neither lungfishes nor amphibians 

 have an annular ligament, which developed in the chondrosteans and 

 went on up the holostean-^teleost branch of the piscine tree; but this 

 is a negative sort of resemblance — as well say that neither group has a 

 chorioid gland; and, we have seen (brook lampreys!) that the structure 

 can be absent in forms whose better-eyed relatives have it. Either ancient 

 lungfishes, ancient amphibians, or both may have had annular ligaments 

 as well as a number of other things. 



Many amphibian features are entirely 'new', and while some of them 

 may serve to link the group with higher forms, none can have any 

 significance for the derivation of the amphibian ocular pattern from 

 anything below. Among such features must certainly be listed the re- 

 tractor and levator bulbi muscles, the extra-ocular glands, the lids, the 

 iris folds (homoiologous, only, with those of elasmobranchs) and pupil- 

 lary nodules of anurans, the loss of the argentea, the secondarv absence 

 of scleral cartilage in some adults and its delayed formation in urodeles, 

 the protractor muscles (at least the dorsal one of anurans) , the fibrous 



