ON THE ORIGIN OF GENERA. 



105 



deserts, the great Sahara and the sands of Arizona and California. 

 There is also a tendency to produce spiny forms in such places • 

 witness the Stellios and Uromastix and Cerastes of the Sahara, 

 the Phrynosomas and horned rattlesnake of Southwestern Amer- 

 ica. The vegetation of every order, we are also informed, is in 

 these situations extremely liable to produce spines and thorns. 



The serpents of the Neotropical region furnisli remarkable 

 illustrations of mimetic analogy. All the species of the genera 

 Elaps, Pliocercus, Erythrolamprus, and many of those of Oxyr- 

 rhopus, Ophibolus and Rhabdosoma are ornamented with black 

 and yellow rings on a crimson ground. The species of all these 

 genera are harmless, except in the case of Elaps, which is venom- 

 ous. We may give for this genus, as the most varied, the follow- 

 ing range of variation in coloration : 



Species a, from Mexico and Central America. (Two species are now added, 1886.) 

 " b, " Brazil, Venezuela. 

 " c, " Central America. 

 " d, " western side of Andes. 

 " h, " Arizona and Sonora. 



Many of the species in the same column are exceedingly simi- 

 lar, and some have little (perhaps nothing) to distinguish them but 

 generic characters. The most similar are almost always from the 

 same sub-region.* These facts are illustrated in Plates III and III a. 



* Similar parallels exist between the Mexican species of Rhadinsea, Conophis, 

 and Erythrolamprus ( = Coniophanes). (Note, 1886.) 



