Then for miles we rode through a beautiful open forest of tall, slender caranda palms, with other trees scattered among them.  Green parakeets with black heads chattered as they flew; noisy green and red parrots climbed among the palms; and huge macaws, some entirely blue, others almost entirely red, screamed loudly as they perched in the trees or took wing at our approach.  If one was wounded its cries kept its companions circling around overhead.  The naturalists found the bird fauna totally different from that which they had been collecting in the hill country near Corumba, seventy or eighty miles distant; and birds swarmed, both species and individuals.  South America has the most extensive and most varied avifauna of all the continents.  On the other hand, its mammalian fauna, although very interesting, is rather poor in number of species and individuals and in the size of the beasts.  It possesses more mammals that are unique and distinctive in type than does any other continent save Australia; and they are of higher and much more varied types than in Australia.  But there is nothing approaching the majesty, beauty, and swarming mass of the great mammalian life of Africa and, in a less degree, of tropical Asia; indeed, it does not even approach the similar mammalian life of North America and northern Eurasia, poor though this is compared with the seething vitality of tropical life in the Old World.  During a geologically recent period, a period extending into that which saw man spread over the world in substantially the physical and cultural stage of many existing savages, South America possessed a varied and striking fauna of enormous beasts—­sabre-tooth tigers, huge lions, mastodons, horses of many kinds, camel-like pachyderms, giant ground-sloths, mylodons the size of the rhinoceros, and many, many other strange and wonderful creatures.  From some cause, concerning the nature of which we cannot at present even hazard a guess, this vast and giant fauna vanished completely, the tremendous catastrophe (the duration of which is unknown) not being consummated until within a few thousand or a few score thousand years.  When the white man reached South America he found the same weak and impoverished mammalian fauna that exists practically unchanged to-day.  Elsewhere civilized man has been even more destructive than his very destructive uncivilized brothers of the magnificent mammalian life of the wilderness; for ages he has been rooting out the higher forms of beast life in Europe, Asia, and North Africa; and in our own day he has repeated the feat, on a very large scale, in the rest of Africa and in North America.  But in South America, although he is in places responsible for the wanton slaughter of the most interesting and the largest, or the most beautiful, birds, his advent has meant a positive enrichment of the wild mammalian fauna.  None of the native grass-eating mammals, the graminivores, approach in size and beauty the herds of wild or half-wild cattle