And now, collecting together all we have learned, let us in conclusion form a rough picture of the history of the fish-world. All over our globe, from pole to pole, and from the Indian Ocean round to the east, back to the Indian Ocean again, is one vast world of waters, with inlets and land-locked seas bordering its margins, and rivers pouring into its depths. In the past ages of the world these rivers and coasts and inlets have varied innumerable times, but the great ocean-mother has always been there to bear the increasingly-varied forms in her bosom, and to enable them to wander where best they could preserve life. And so from their beginning, when they were probably as feeble as the lancelet, these earliest and simplest backboned animals with their two pair of limbs as yet very variable both in their position and shape, have been spreading far and wide over the watery three-quarters of the globe. We have seen how the enamel-scaled fish had their time of glory, but were not able to hold their ground, because they were not agile and fish-like enough to escape their foes; and how the sharks by their strength and boldness remain monarchs of the sea to the present day. Then we have seen that in old chalk seas the new and active race of bony fish appear in force; some like the herring and the carp, with air-bladders, which had openings like the enamel-scaled fish, and these can dart from heights to depths; while others had closed air-bladders, and these remain with most ease at one level, and can sometimes, if necessary, use the gas in their bladder for breathing, if they are oppressed with muddy water; and lastly, some, such as the dorado, have lost their air-bladder altogether, and gain in freedom of action what they lose in lightness and buoyancy. And during the ages that have passed since this bony race began, different branches each in their own way have thrown out curious weapons and developed strange organs to help them in the battle of life, so that now we have deep-sea fish carrying their own light; fish with distensible stomachs swallowing prey larger than themselves; fish with large air-bladders and long arm-fins springing out of their own element and floating in air; angling-fish, walking-fish, clinging-fish, and hiding-fish; and even those whose shape is distorted, like the sole, to enable them to hide and hunt in safety; while, when the sea is full, we find new varieties pressing their way into every river and tiny stream, and even overland into enclosed waters. Nay! when we descend into the recesses of the earth and visit the underground pools of the dark caverns of Kentucky, there we come upon fish which have found a refuge in eternal darkness, and have lost not only the power of sight but actually the eyes themselves.