72 MOSTLY MAMMALS 
goose-like chaja (pronounced chahda), or horned screamer, 
which takes its English name from the spur on its wing 
and its loud cry, the latter being sometimes heard when 
the bird is so high in the air as to be almost or quite 
invisible. The long-legged seriema, which stalks over the 
plains in the manner of the African secretary-bird, is 
like-wise a very characteristic type. Among characteristic 
South American reptiles may be mentioned iguanas (a 
name often applied incorrectly to lizards from other parts 
of the world) and caimans; the latter being a group of 
alligators distinguished by having an armour of bony 
plates on the under as well as on the upper surface of 
the body. The huge horned frogs (Ceratophrys) are like- 
wise distinctive of the country among the batrachians. 
Such are a few of the leading features of the existing 
fauna of South America, which are sufficient to show how 
totally different is the animal life of this country from 
that of all the rest of the world. If, however, we go back 
to the later geological periods of the earth’s history, we 
shall find that this peculiarity and distinctness of the 
South American fauna was even more intensified than at 
the present day, this being largely due to the circumstance 
that at one time the isthmus of Darien seems not to have 
existed, so that the northern and southern portions of the 
New World were disconnected. Since the time when a 
connection was formed between the two continents, their 
faunas have, however, naturally tended to blend together, 
and hence at the present day, and during the Pleistocene 
period, the animals of South America are less sharply 
differentiated from those of the northern half of the con- 
tinent than would have been the case had the isthmus 
of Darien not been formed, It is further interesting to 
note that during the Tertiary period there appears to have 
