The reason of this wonderful effectiveness of the animal machine is obvious. See how this machine has been gradually perfected throughout infinite ages, especially in birds. During the whole geological history of the earth this machine has been steadily improving in structure of skeleton, energy of muscle, and rapidity of combustion of fuel, by struggle for life and survival of only the swiftest, the most energetic, and the hottest-blooded, until an almost incredible intensity is reached in birds. Moreover, in them everything is sacrificed to the supreme necessity of flight. Viscera, skeleton, legs, head, all are made as small and light as possible to make room for the great pectoral muscles working the wings. Add to this the exquisite structure of the wings and feathers, adapting them for the greatest effectiveness; and we must admit that a bird is an incomparable model of a flying-machine. No machine that we may hope to devise, for the same weight of machine, fuel, and directing brain, is half so effective. And yet, this machine thus perfected through infinite ages by a ruthless process of natural selection, reaches its limit of weight at about fifty pounds! I said, "weight of machine, fuel, and directing brain." Here is another prodigious advantage of the natural over the artificial machine. The flying animal is its own engineer, the flying-machine must carry its engineer. The directing engineer in the former (the brain) is perhaps an ounce, in the latter it is one hundred and fifty pounds. The limit of the flying animal is fifty pounds. The smallest possible weight of a flying-machine, with its necessary fuel and engineer, even without freight or passengers, could not be less than three or four hundred pounds.
Now, to complete the argument, put these three indisputable facts together: 1. There is a low limit of weight, certainly not much beyond fifty pounds, beyond which it is impossible for an animal to fly. Nature has reached this limit, and with her utmost effort has failed to pass it. 2. The animal machine is far more effective than any we may hope to make; therefore the limit of the weight of a successful flying-machine can not be more than fifty pounds. 3. The weight of any machine constructed for flying, including fuel and engineer, can not be less than three or four hundred pounds. Is it not demonstrated that a true flying-machine, self-raising, self-sustaining, self-propelling , is physically impossible?
6. Application to a Swimming-Machine.—But is there not a way of escape from the toils of this inexorable logic? We have said the limit of the weight of a flying animal is about fifty pounds. The limit for a walking animal is much higher, probably several tons. For a swimming animal there is no limit of weight and size, because the water sustains the weight, and therefore the