A bird with fully formed wings has a survival value through being able to fly away from land enemies. But if it evolved from a reptile by Natural Selection, then each stage between the useful Reptilian fore-leg and the useful wing must have had a special advantage over the stage immediately preceding it. There must have been an advantage in the fore-leg getting stumpy, then in its getting stumpier, then its getting so stumpy that the beast couldn’t use it at all. And this must be true of every change in all the millions of tiny evolutionary changes proceeding through æons of time. All the way along, from the first signs of something which _later on_ will be an advantage to the mature type, through myriads of generations, from the first origins when the organ was as yet rudimentary to the last when it was perfected, every step must have had a survival value over the last. And this must apply not only to broad cases, such as the reptile’s fore-leg turning into the bird’s wing, but to every one of innumerable organs and to every part of each organ. Otherwise the theory breaks down. The implications then of Natural Selection as the blind agent of development, “give one furiously to think.” Merely stated roughly as I have done here, they shake the ordinary man’s confidence in it. But when we come to ordered proofs against it, we shall find those proofs conclusive. To these I now turn. THE ARGUMENTS AGAINST NATURAL SELECTION When one has to examine any proposition and see whether it be true or no, two radically distinct forms of reasoning present themselves to the mind. (A) You may find the thing asserted to be in itself impossible, granted certain self-evident principles of thought. For instance, a man who died on the 16th of the month cannot have died of poison taken on the 20th of the month. Or again, the sum of certain payments cannot be less than some one of those payments. This method is called the _a priori_ method. (B) The other approach made by reason to see whether a theory is true or false is the experimental one, that of positive evidence. You test, by the positive evidence at your disposal, whether the thing affirmed has really taken place or not. Sometimes the first of these methods is conclusive, in which case one has no reason to go any further. Sometimes, and more usually, the second is conclusive, and there is no opportunity or occasion to apply the first. For instance, if we are told that John Jones forged the will of a man who was born after John Jones’s death we know _a priori_ that the story is nonsense. But if John Jones is said to have forged the will of a man who died while John Jones was still alive, then we must go into the evidence of handwriting and all the rest of it.