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is that amongst fishes we find some old geological types still preserved in a few of the large rivers of the world.
To illustrate more clearly the theory I have advanced, I will take a supposititious case. In the southern states of America there is reason to suppose that since the glacial period there has been a great variation in the species of the fresh-water mollusk genus Melania, and in different rivers there are distinct groups of species. Now let us suppose that the glacial period were to return, and that the icy covering, gradually thickening in the north, should push down southward as it did once before.
The great lakes of North America would be again filled with ice, and their inhabitants destroyed. As the ice advanced southward, the inhabitants of one river-system after another would be annihilated, and many groups of Melania entirely destroyed. On the retreat of the ice again the rivers and lakes would reappear, but the varieties of animals that had been developed in them would not, and their places would be taken by aquatic forms from other areas, so that the number of species would be thereby greatly reduced, and wide-spreading forms would be freed from the competition of many improved varieties.
Viewed in this light, the similarity of fresh-water productions all over the world, instead of being a difficulty in the way of the acceptance of the theory of natural selection, becomes a strong argument in favour of its truth; for we perceive that the number of marine, terrestrial, and fresh-water animals is in proportion to the more or less continuous development that was possible under the different conditions under which they lived.
The same line of argument might be used to explain the much greater variety in some classes of terrestrial animals than in others. The land has often been submerged in geological history, and the classes that were best fitted to escape the impending catastrophes would be most likely to preserve the varieties that had been developed. The atmosphere has always been continuous, and the animals that could use it as a highway had great advantages over those that could not, and so we find the slow-moving terrestrial mollusks few in number compared with the multitudinous hosts of strong-flying insects; similarly, the mammals are far outnumbered by the birds of the air, that can pass from island to island, and from country to country, unstopped by mighty rivers or wide arms of the sea.
Iguanas and lizards-Granada–Politics—Revolution-Cacao cultiva
tion-Masaya—The Lake of Masaya-—The volcano of MasayaOrigin of the lake basin.
The road passed along a sandy ridge only a little elevated above the waters of the lake, and the ground on both sides was submerged. As we travelled on we were often startled by hearing sudden plunges into the water not far from us, but our view was so obstructed by bushes that it was some time before we discovered the
At last we found that the noise was made by large iguana lizards, some of them three feet long, and very bulky, dropping from the branches of trees, on which they lay stretched, into the water. These iguanas are extremely ugly, but are said to be delicious eating, the Indians being very fond of them. The Carca Indians, who live in the forest seven miles from Santo Domingo, travel every year to the great lake to catch iguanas, which abound on the dry hills near it. They seize them as they lie on the branches of the trees, with a loop at the end of a long stick. They then break the middle toe of each foot, and tie the feet together, in pairs, by the broken toes, afterwards sewing up the mouth of the poor reptiles, and carrying them in this state back to their houses in the forest, where they are
kept alive until required for food. The raccoon-like
pisoti” is also fond of them, but cannot so easily catch them. He has to climb every tree, and then, unless he can surprise them asleep, they drop from the branch to the ground and scuttle off to another tree. I once saw a solitary “pisoti” hunting for iguanas amongst some bushes near the lake where they were very numerous, but during the quarter of an hour that I watched him, he never caught one. It was like the
puss in the corner.” He would ascend a small tree on which there were several; but down they would drop when he had nearly reached them, and rush off to another tree. Master “pisoti,” however, seemed to take all his disappointments with the greatest coolness, and continued the pursuit unflaggingly. Doubtless experience had taught him that his perseverance would ultimately be rewarded: that sooner or later he would surprise a corpulent iguana fast asleep on some branch, and too late to drop from his resting-place. In the forest I always saw the “pisoti” hunting in large bands, from which an iguana would have small chance of escape, for some were searching along the ground whilst others ranged over the branches of the trees.
Other tree-lizards also try to escape their enemies by dropping from great heights to the ground. I was once standing near a large tree, the trunk of which rose fully fifty feet before it threw off a branch, when a green Anolis dropped past my face to the ground, followed by a long green snake that had been pursuing it amongst the foliage above, and had not hesitated to precipitate itself after its prey. The lizard alighted on its feet and hurried away, the snake fell like a coiled-up watch-spring, and opened out directly to continue the pursuit; but, on the spur of the moment, I struck at it with a switch and prevented it. I regretted afterwards not having allowed the chase to continue and watched the issue, but I doubt not that the lizard, active as it was, would have been caught by the swiftgliding snake, as several specimens of the latter that I opened contained lizards.
Lizards are also preyed upon by many birds, and I have taken a large one from the stomach of a great white hawk with its wings and tail barred with black (Leucopternis ghiesbreghti) that sits up on the trees in the forest quietly watching for them. Their means of defence are small, nor are they rapid enough in their movements to escape from their enemies by flight, and so they depend principally for their protection on their means of concealment. The different species of Anolis can change their colour from a bright green to a dark brown, and so assimilate themselves in appearance to the foliage or bark of trees on which they lie. Another tree-lizard, not uncommon on the banks of the rivers, is not only of a beautiful green colour, but has foliaceous expansions on its limbs and body, so that even when amongst the long grass it looks like a leafy shoot that has fallen from the trees above. I do not know of any lizard that enjoys impunity from attack by the secretion of any acrid or poisonous fluid from its skin, like the little red and blue frog that I have already described, but I was told of one that was said to be extremely venomous. As, however, besides the repute of giving off from the pores of its skin poisonous secretion, it was described to be of an inconspicuous brown colour, and