BIRDS OF SOUTHWESTERN MEXICO. 7 



no driuking-water but that which they draw out of holes dug in the 

 sand. 



"From that it can be easily explained how the number of animal 

 forms, as well as the vegetable, decreases perceptibly in proportion as 

 you advance from the Atlantic to the Pacific. 



" lu a zoological point of view, the preceding division into three 

 regions is modified in this sense, that the central part or mean does not 

 present any distinguishing forms which can characterize it. 



"A line drawn on the map through the villages of Santa Maria, Chima- 

 lapa, and San Juan Guichicovi would indicate quite correctly enough 

 the boundary-lines between the two zoological provinces or regions 

 which divide the isthmus, and almost that of a division of the waters 

 which flow to the Atlantic and the Pacific. Several places situated on 

 the crest of this line present, as might be expected, a mixture of forms 

 belonging to each littoral; thus, in the neighborhood of Barrio, Conurus 

 aztec and petzi, Chrysotis autumnalis and alhifrons, PsilorMnus morio, 

 and Calocitta formosa, &c., are found together. 



" It is to be noticed that, while the species belonging to the western 

 province seldom or never leave it to spread in the opposite direction, 

 several of those in the eastern province advance, on the contrary, to 

 w^ithin a short distance of the shores of the Pacific. To quote as ex- 

 amples : Turdus grayi, Attila citreopygia, Muscivora mexicana, Bhyncho- 

 cyclus cinereiceps, Oncostoina cinereigulare^ GhiroxipMa linearis, Chrysotis 

 levaillanti, Fteroglossus torquatus, Penelope purpurascens, Grax glohicera, 

 Tinamus sallcei, &c.; all of them species whose place of development is 

 without contradiction iu the limits of the Atlantic region, but which 

 are found in the immediate neighborhood of the Pacific, (Santa 

 Efigenia). 



" The difference in the level of the ground, which exerts elsewhere in 

 Mexico such a great influence over the geographical distribution of ani- 

 mal species, only exists in a slight degree in the Isthmus of Tehuantepec ; 

 one of the culminating points of this territory, the Cerro de Mazahua, 

 is not elevated probably more than from 500 to 2,800 feet above the level 

 of the sea. We must not, therefore, expect to find in the isthmus prop- 

 erly so called any of the indigenous species which elsewhere charac- 

 terize alpine regions. The few species of that region which are found 

 in my collections have been gathered out of the isthmus ; some in the 

 Sierra of Oaxaca, others in the mountains of Gineta and of Zapotitlan. 



" If, in order to establish a sort of parallel between the two ornitho- 



