“I couldn’t tell exactly what beast it was,” said Perry, in the course of the conversation, “because I couldn’t see that the skull looked anything at all like an elephant’s.” “You’re right, it doesn’t,” his uncle agreed. “I couldn’t see how it was an ancestor, then. I don’t quite see, even yet. An elephant has tusks and a trunk. This little Moeritherium hasn’t either, so far as I can make out.” “That’s because the animal is so far back in the line of development,” the scientist reminded him. “He isn’t in a direct line, but more like a first cousin of what the ancestral elephant must have been, although we haven’t found any specimens of him yet. As for the trunk—well, it’s true there isn’t any sign of that, the eyes are too far forward. But the tusk question is interesting. Do you know, Perry, which are the teeth that the elephant has developed into tusks?” The boy thought for a moment. “No, Uncle George, I don’t,” he said. “Until this minute I never stopped to think that an elephant’s tusks were teeth.” “What did you think they were? Horns?” “I—I hadn’t ever thought,” stammered Perry, confused. “I just thought of them as tusks.” “They are the incisors,” the scientist replied. “Now, in Moeritherium, you can see that the second incisors are developed both in the upper and lower jaws.” He held out the skull to the boy. “Yes,” Perry answered, “that’s easy enough to see.” “Now in the skull of Paleo-mastodon, as I explained to you fully last night, there were rudimentary first incisor teeth. You remember that?” “Yes,” answered the boy. “And the elephant hasn’t any first incisors at all.” “Oh!” exclaimed Perry, “I’m beginning to get hold of the idea. The Moeritherium shows how the tusks started.” “In a way, though not directly. Moeritherium never developed tusks from his teeth. Now you get Antoine, sometime, to show you the details of the teeth of a typical mammal. You’ll find that there ought to be forty-four. Although he had little tusks, Moeritherium kept a fairly complete set of teeth, while Paleo-mastodon, in order to get real solid tusks, was compelled to sacrifice all his incisors but four. You’ll find that the important things in paleontology are teeth and feet.” “That’s what I always strike in books. Just why is that, Uncle George?” “Think a bit, and figure it out for yourself.” Perry stared at the Moeritherium skull, and tried to picture the development of life in primitive times, millions of years before the first man walked the earth. “I suppose,” he said, after quite a long pause, “it’s because the two main things an animal had to do was to eat and to avoid being eaten. Animals with weak teeth had to give place to animals with better teeth when the food got harder to chew, and animals that were likely to be eaten had to find ways of escape. The ones with poor feet were caught and eaten, the swift ones got away.”