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The answer, as I believe, is, that mammals have not been able to migrate, whereas some plants, from their varied means of dispersal, have migrated across the vast and broken interspace.
No doubt the males of some obscurely-coloured birds fight desperately together, but it appears that when sexual selection has been highly influential, and has given bright colours to the males of any species, it has also very often given a strong tendency to pugnacity. We shall meet with nearly analogous cases when we treat of mammals.
It had already found a place among the herds of apes and other social mammals; in a similar manner, but with wider scope, it was already present in the most primitive communities and among the hordes of the least advanced savages.
Only the birds and mammals have warm blood, and of these the birds are distinctly the warmer. Whereas the temperature of the mammals runs from about ninety-eight to a hundred degrees Fahrenheit, that of birds lies somewhere between one hundred and five degrees and a hundred and ten. Creatures which are warmer than their surroundings must have some protection against chilling.
The remains of these interesting early mammals, restricted, as they generally are, to jaws and teeth and a few other bones that cannot in themselves be too confidently distinguished from those of certain reptiles, may seem insufficient to enable us to form a picture of their living forms. In this, however, we receive a singular and fortunate assistance.
"Yes, to be sure; the great African lion itself is nothing but the largest and strongest of all the cat tribe. Didn't you know that?" "I thought the lion was a beast by itself; but, at all events, it is the king of mammals?" "It is rightly thought to be the strongest of all the carnivora: its head, which it carries upright, and its beautiful mane, give it a majestic appearance.
Rudimentary organs sometimes retain their potentiality: this occasionally occurs with the mammae of male mammals, which have been known to become well developed and to secrete milk. So again in the udders of the genus Bos, there are normally four developed and two rudimentary teats; but the latter in our domestic cows sometimes become well developed and yield milk.
No doubt some few anomalies occur in this great archipelago, and there is much difficulty in forming a judgment in some cases owing to the probable naturalisation of certain mammals through man's agency; but we shall soon have much light thrown on the natural history of this archipelago by the admirable zeal and researches of Mr. Wallace.
The soil of the district appeared too dry; at this season of the year I had noticed, in other parts of the country, that mammals and birds resorted to the more humid areas of forest; we therefore proceeded to explore carefully the low and partly swampy tract along the coast to the north of Santa Cruz.
Cherrie and Miller had spent a week there collecting mammals and birds prior to my arrival at Asuncion. They were veterans of the tropics, hardened to the insect plagues of Guiana and the Orinoco. But they reported that never had they been so tortured as in the Chaco.
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