literal flood of waters streams out of the sides of the mouth, the "sea butterflies" are strained off therefrom, the savory morsels being retained by the fringed edges of the baleen-plates, and thereafter duly swallowed as food.
An interesting speculation yet remains, however, regarding the origin and first development of these peculiar whalebone-structures. Advocates of the doctrine which assumes that animal forms and their belongings arise by gradual modifications of preëxistent animals may be reasonably asked to explain the origin of the baleen-plates of the whales. Let us briefly hear what Mr. Darwin, as the spokesman of the party, has to say in reply to such an inquiry. Quoting a remark of an opponent regarding the whalebone, Mr. Darwin says, if the baleen "'had once attained such a size and development as to be at all useful, then its preservation and augmentation within serviceable limits would be promoted by natural selection alone. But how to obtain the beginning of such useful development?' In answer," continues Mr. Darwin (in his own words), "it may be asked, why should not the early progenitors of the whales with baleen have possessed a mouth constructed something like the lamellated beak of a duck. Ducks, like whales, subsist by sifting the mud and water; and the family (of ducks) has sometimes been called Criblatores, or sifters." Mr. Darwin's reference to the duck's bill is peculiarly happy. The edges of the beak in these birds are fringed with a beautiful series of horny plates named lamellæ, which serve as a straining apparatus as the birds grope for their food amid the mud of ponds and rivers. These plates are richly supplied with nervous filaments, and doubtless also some as organs of touch. Mr. Darwin is careful to add that he hopes he may not "be misconstrued into saying that the progenitors of whales did actually possess mouths lamellated like the beak of a duck. I only wish to show," he continues, "that this is not incredible, and that the immense plates of baleen in the Greenland whale might have been developed from such lamellæ by finely graduated steps, each of service to its possessor."
In these last words, which we have italicized, lies the strength of Mr. Darwin's hypothesis. Nature will preserve and develop useful structures alone, and will leave the useless and unneeded to perish and decay. This, indeed, is the keynote of natural selection. Mr. Darwin next proceeds to examine in detail the plates and lamellæ in the bill of a shoveler duck. He describes the horny plates, one hundred and eighty-eight in number, which "arise from the palate, and are attached by flexible membrane to the sides of the mandible." He further notes that these plates "in several respects resemble the plates of baleen in the mouth of a whale." If the head of a shoveler duck were made as long as the head of a species of whale in which the baleen-plates are only nine inches long, the duck's lamellæ would be six inches in length. The head of the shoveler is about one eighteenth
