300 PROCEEDINGS OF THE ACADEMY OF 



Ctenosaura. 'The serpents are Opkibolus splendidus, a new Crotalus, Masticophis 

 testaceus ? a Rio Grande Nerodia, Arizona elegans, Scotophis Emoryii, Lamprosoma 

 episcopum, etc., or species very closely allied to them. 



The Zoology of the east side of the gulf of California is not sufficiently well 

 known to allow a satisfactory comparison with that of Cape St. Lucas ; it is 

 however probable that the Gila fauna does not extend as far down as the lati- 

 tude of the Cape, being displaced by the northward extension of the 

 fauna of Western Mexico. Even at Guaymas, species of birds and reptiles occur, 

 of genera different from those of the United States, as for instance the genus 

 Dryophis among the serpents. 



It may safely be considered as very probable, that additional species of the 

 Gila and Colorado regions will hereafter be detected at the Cape, and that a 

 closer examination of the former localities will bring to light several of the 

 species for the first time noticed in the Cape collection of Mr. Xantus. 



What the causes are which have produced this peculiar distribution of ani- 

 mal life on the Cape, it is at present impossible fully to elucidate. The moun- 

 tain crests which extend longitudinally along the peninsula might form an 

 impassible barrier to the passage of species from one coast to the other, but as 

 there appears to be no greater obstacle to the extension southward to Cape St. 

 Lucas from the coast region of Upper California, than from the mouth of the 

 Colorado along the east side of the peninsula, we would expect to find a much 

 greater mixture of species at the Cape than really exists. No information is at 

 present at our command as to the zoology of the interior of the peninsula. It is, 

 however, quite probable that the narrow vallies enclosed between the moun- 

 tainous sides of the peninsula may have species widely different from either 

 those of the Cape itself, and of Upper California, and more analogous to those 

 of Mazatlan and its vicinity. 



The region in which Mr. Xantus obtained the birds hereafter enumerated, is 

 one which at first sight would not seem a very promising field for exploration. 

 The shore is sandy for about a quarter of a mile inland, whence a cactus desert 

 extends for a width of about six miles up to the high mountains on the West and 

 North. The Cereus giganteus is a prominent feature in this peculiar vegetation, 

 rising occasionally to a height of sixty or more feet. The ground is covered for 

 miles with a saline efflorescence, painful to the eye, into or through which the 

 feet sink to a considerable distance. There is no fresh water nearer than San 

 Jose, a distance of twenty-eight miles. The region, though in the spring and 

 summer inhabited almost exclusively by land birds, is said in the rest of the 

 year to be the resort of innumerable water fowl and waders, among which Mr. 

 Xantus will doubtless find many rare species. 



Before proceeding to an enumeration of the summer birds of Cape St. Lucas, 

 it may be well to state that they illustrate in a remarkable degree the law de- 

 rived from an examination of large series of specimens in the Smithsonian 

 museum, and frequently referred to in the ninth volume of the Pacific R. R. 

 Report ; namely, that whenever species have a wide range in latitude as resi- 

 dent birds or as summer visitors, the farther North the species is found breed- 

 ing, the larger it is, and vice versa. The same principle applies, though in less 

 marked degree, to an increasing altitude in the same latitude. The difference 

 in size between the same species of bird breeding at Cape St. Lucas and in 

 the Colorado Valley, or in the more northern Rocky Mountains, is very strik- 

 ing, so much so as readily to induce the impression of a difference in the 

 species. 



The following table will illustrate more fully what has been said in regard to 

 the geographical distribution and character of the species. It will be seen that 

 all the characteristic land species of the Cape (all supposed to be new except- 

 ing Colaptes chrysoides) are exceedingly abundant, breeding in large num- 

 bers. 



[Nov. 



