THE METHOD OF CEEATION OF OEGANIO FORMS. 197 



or encouraged by suppression of use of the branchial and encourage- 

 ment of use of the pulmonary organs, or the reverse. 



The aquatic respiration of tadpoles may be indefinitely pro- 

 longed by preventing their access to the surface, and it is known 

 that in nature the size or age of the larva at time of metamorphosis 

 may vary much in the same species. If perennibranchiates {Siren, 

 e. g.) are deprived of their branchiae, they will aerate blood by 

 the lungs exclusively, and there is no reason to doubt that by use 

 of these, and disuse of the branchia3, aerial respiration might be- 

 come the habit of the animal. It is also easy to perceive that geo- 

 logic changes would bring about a necessity for precisely this change 

 of habit. This occurred in the period of the coal measures, where 

 large fresh-water areas were desiccated, and it was precisely at this 

 period that many air-breathing Batrachians originated and had a 

 great development. 



i3. The rattle of the Rattlesnake. — Nearly all the larger harm- 

 less snakes which live on the ground have a habit of throwing the 

 end of the tail into violent vibrations when alarmed or excited, 

 with the view of alarming a supposed enemy. Among Coronelline 

 snakes, Ophibolus triangulus possesses it ; among the water snakes, 

 Tropidonotus sipeclon. In the typical Colubrine group the black 

 snake, Bascanium constrictor, is an example ; Pityophis sayi also 

 shakes the tail violently. The copperhead {Ancistrodon contor- 

 trix) and the moccasin {A. piscivorus) (fide Giinther) have the 

 habit in a marked degree. Among the rattlesnakes it is a means 

 of both warning and defense, in connection with the rattle which 

 they carry. 



In the structure of the end of the tail of harmless snakes, we 

 see a trace of the first button of the rattle in a horny cap that covers 

 the terminal vertebrae. In the venomous genera, it is conspicuous 

 in Lachesis especially, reaching a considerable length and having 

 a lateral groove. In the plate-headed rattlesnakes {Crotalus) this 

 corneous cap is inflated into a button with lateral groove, and in 

 some of them possesses only one or two buttons or joints. In the 

 perfected rattlesnakes {Caudisona) not only are the segments nu- 

 merous and inflated, but a number of the terminal caudal vertebra? 

 are greatly enlarged vertically, and co-ossified into a mass.* This 

 is important from the fact that the rattlesnakes are the most spe- 



* Soc good figures of this structure in " Zcitsehr. f. wissensch. Zoologic," viii, 

 Tab. 12. 



