When the Nile overflowed its banks in ancient times, and caused the young frogs to swarm up as a pest upon the Egyptians, the same law of life was operative in that land, as when warm thunder-showers pelt the earth with us in the summer season, causing hundreds and thousands of these batrachians to come out of the gritty waysides, and swarm along our highways and by-ways, leading ignorant and thoughtless people to suppose that they have rained down from the sky. The simple fact is, that the earth was commanded to bring them forth, and that great mother of all vegetable and animal life is obeying the command to-day, just as she did in the beginning. One of the greatest errors that science has yet committed, or rather that scientific men have stumbled upon, is the theory that all living forms have appeared but once in time and place, and that they have thence diffused themselves, in pairs, throughout the globe, as from specific centres of origin. In the primeval oceans, whenever and wherever the environing conditions of matter were the same or identical, the like living forms made their appearance and flourished for hundreds and thousands of years, and finally disappeared, in a fossilized state, as their environing conditions were changed. They came not genetically--as in pairs--but thronged the seas in thousands and millions as the divine edict went forth. As another conclusive proof, to our mind, of the existence of this law of life, we instance the case of the mango-tree growing in the West India Islands, especially along the sea-shore, where it becomes the natural _habitat_ of the oyster. It is the belief of some ignorant persons that the oyster climbs these trees and deposits its spawn or "spat" upon the extreme limbs of the same as they bend down toward the water. This is manifestly an error, and belongs to the same class of fallacies as the common impression that toads rain down from the sky. The smaller mango-trees growing about the bays and inlets of these islands, furnish, as we have said, a natural _habitat_ for the oyster, and as the salt sea-spray washes their roots and the bark of their trunks, the long thin-shelled oysters of that region make their appearance thereon without the presence of spawn, just as they do when old oyster-shells are dumped along our sand-banks in New England. On these dumped shells oysters will be produced abundantly, simply because the conditions are favorable, and not in consequence of the presence of "spat." Oysters have little, if any, locomotive power, and can no more climb the mango-tree than they can scale the cliffs of the Azores. The reason why they hang in pendent clusters from the extreme boughs of the mango in the West India Islands is, that these boughs are sprayed upon by the rippling waters, and the environing conditions being favorable, the indifferent oyster of that region makes its appearance.