Of course, even where they didn't die with their boots on, so to speak, as so many of them did in those lawless days, there came a time for each monster, in the order of nature, when he drew his last breath. But what seems so strange is that all these monsters--the biggest and strongest of them--entirely disappeared and left no descendants![6] The whole of the mystery has not been unravelled yet, even by the wise men of science, but still they have learned a good deal. For one thing, they know that most of the reptiles and the fish-lizards disappeared because so much of the land where they lived went dry. They had to get a new boarding-place, and there wasn't any to get! Another thing was that these big fellows, although they _were_ so big, and got along finely while everything was just so, had so little brain they couldn't change their habits to meet new conditions, as our closer and cleverer cousins, the mammals, did. Why, do you know that one of these monsters, who was twenty-five feet long if he was an inch, and twelve feet high, had a brain no bigger than a man's fist? All the monsters of those days were like that--tons of bone and muscle, but a very small supply of brains. [6] That is to say, no descendants worthy of them. It is now thought some of the modern reptiles may be degenerate descendants of the big reptiles of old. So when things went against them, they just had to give up, and, like a queer dream, they faded away. But their history makes one of the most interesting chapters in the whole wonderful story of the dust. Of all the live stock that have fed on the great world-farm and helped enrich it with their bones, these animals were surely the strangest that ever were seen! HIDE AND SEEK IN THE LIBRARY "But since these monsters passed away many millions of years ago, and all that is usually found is a piece of them here and there, how do the men of science know so much about them--how they looked, and how they ate, and how they treated one another?" That's a good question. It _does_ seem strange. Why, to hear them talk, you'd suppose these men, learned in ancient bones, had actually _met_ the monsters! And, speaking of meeting them, I must tell you a little story. It's a good story and it will answer your question. Baron Cuvier, one of the most famous of the paleontologists, awoke from a deep sleep to see standing by his bed a strange, hairy creature with horns and hoofs. And it said: "Cuvier! Cuvier! I have come to eat you!" But the baron, taking in the form of the monster at a glance, only laughed. "Horns and hoofs? You can't. You're a grain-eater!"