Why are there not water-babies?

"Now little Ellie was, I suppose, a stupid little girl; for she only asked the same question over again. "But why are there not water-babies?" I trust and hope that it was because the professor trod at that moment on the edge of a very sharp mussel, and hurt one of his corns sadly, that he answered quite sharply, forgetting that he was a scientific man, and therefore ought to have known that he couldn't know; and that he was a logician, and therefore ought to have known that he could not prove a universal negative—I say, I trust and hope it was because the mussel hurt his corn, that the professor answered quite sharply: "Because there ain't."
Charles Kingsley, The Water-Babies