76 THE REPTILES OF THE GALAPAGOS ISLANDS. 



cal affinities, and they must have come from nearly allied 

 forms, not from the same form. It may be that both of 

 these genera developed on the same island, the arid belt 

 near the shores evolving the cactus-eating Conolophus 

 while Amblyrhynchus made its food of seaweeds. Or it 

 may have been that Amblyrhynchus developed on one or 

 more of the islands on which there was no alternative for the 

 seaweed, whence the lizard has reached other localities in 

 which it now occurs. How these saurians became pos- 

 sessed of the vegetarian habit is a question to which our 

 only answer is conjecture. Its inheritance from herbiv- 

 orous mesozoic progenitors that might have existed is not 

 to be seriously considered. While it may have been the 

 case that allied species on the mainland also to some extent 

 fed on plants, it is more likely that scarcity of animal food 

 rather suddenly brought upon them, whether through emi- 

 gration or otherwise, compelled a change of diet. Such 

 achange would be complete in a single generation ; where- 

 as more gradual diminution in the supply of animals might 

 induce or permit adaptation, by reduction in size or needs, 

 to correspond with the conditions. Conolophus with its 

 feeding habits could only develop in such places as now 

 harbor it, the higher of the islands, those surrounded by 

 the cactus-bearing arid belt and possessing the fertile upper 

 plateaus. By this fact it is restricted to a few of the isl- 

 ands. But Amblyrhynchus is equally at home on any of 

 the islands with sufficient shoal water around them for the 

 production of the seaweeds. It may have started on one 

 of the islands that have no fertile upper belts, which are 

 not high enough to arrest the moisture needed for vegeta- 

 tion. However it reached such a territory it would be 

 obliged to depend on the beaches for subsistence, and from 

 such a place it might spread over the entire archipelago. 



The determinations Dr. Baur has reached in his studies 

 of the genera Testudo and Tropidurus are the following : 



