man's ancestors were quadrupeds, and the time during which he has walked upright is insignificant compared with the time during which they walked on all fours. The structures developed in his ancestors and not yet modified to suit his new posture should be expected to hold anomalous relations. So far as known, the general distribution of valves in the veins is the same in man as in the mammals near him, and when he is placed back on all fours the arrangement of the valves is perfectly intelligible. The veins of the limbs, the jugular and intercostal veins, then carry blood upward; and the venæ cavæ and other valveless veins are horizontal and have no need of valves. Many important facts of a pathological nature are accounted for by the theory of imperfect adjustment of bodily structure and posture.[1] This explanation of them is so striking that Clevenger irreverently suggests that the original sin of man may have been the act of getting up on his hind legs.[2]
When a theory has thus assimilated all the groups of facts related to those from which it sprang and which are unintelligible without it, it has fulfilled the philosophical requirements of a true theory. But every great generalization opens more problems than it closes. This has been true in astronomy, physics, chemistry, and geology, and is true of biology. There are now numberless questions to be answered in biology which could not even be raised without the theory of descent. An illustration may be drawn from the case already cited. Some of the cephalic veins have no valves, but should have them if the explanation is true; the azygos vein has rudimentary valves, but does not need them in the quadrupedal state. These facts become new problems and require subsidiary explanations. By regarding some of the imperfect valves as obsolescent and others as nascent, some disappearing because they are no longer useful and others appearing where they are needed, the exceptions are mostly removed. Frequently such exceptions are not simply accounted for under the theory, but form some of its most striking proofs.[3] Fruitfulness in furnishing problems for solution, instead of indicating weakness, proves the strength of a theory. Alchemy, the Ptolemaic astronomy, and the doctrine of special creation alike, could not lead to a thousandth part of the scientific activity that has followed in the wake of the theories that supplanted them, because they furnished no way of approach to the numberless special problems.
The irresistible power of a true theory rests in the end in the
