THE LOGIC OF ORGANIC EVOLUTION. 387 



man's ancestors were quadrupeds, and the time during which he 

 has walked upright is insignificant compared with the time dur- 

 ing 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 vense cava? 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 imper- 

 fect adjustment of bodily structure and posture.* This explana- 

 tion 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.f 



When a theory has thus assimilated all the groups of facts re- 

 lated 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, chemis- 

 try, and geology, and is true of biology. There are now number- 

 less 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 im- 

 perfect valves as obsolescent and others as nascent, some disap- 

 pearing because they are no longer useful and others appearing 

 where they are needed, the exceptions are mostly removed. Fre- 

 quently such exceptions are not simply accounted for under the 

 theory, but form some of its most striking proofs. J Fruitfulness 

 in furnishing problems for solution, instead of indicating weak- 

 ness, 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 fol- 

 lowed 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 



* Piles, prolapsus uteri, inguinal hernia, etc. f Trout of Yellowstone Park. 



\ For a fine illustration see Wallace, Geographical Distribution of Animals, vol. i, pp. 

 209-214. 



