48 Evolution and Adaptation 



common plan in animals is the consequence of a common 

 descent. " I cannot see the necessity for such a conclusion, 

 and I certainly should unhesitatingly deny that the common 

 plan of the Gothic churches depended on a common archi- 

 tect. The illustration is, however, not perfect, because the 

 influence of the mediaeval school of stone-cutters on its wan- 

 dering apprentices is well known." 



Fleischmann adds that if the descent theory is true we 

 should expect to find that if a common plan of structure is 

 present in one set of organs, as the limbs, it should be pres- 

 ent in all other organs as well, but he does not add that this 

 is generally the case. 



The weakness of Fleischmann's argument is so apparent 

 that we need not attempt an elaborate refutation. When he 

 says there is no absolute proof that the common plan of 

 structure must be the result of blood-relationship, he is not 

 bringing a fatal argument against the theory of descent, for 

 no one but an enthusiast sees anything more in the explana- 

 tion than a very probable theory that appears to account for 

 the facts. To demand an absolute proof for the theory is to 

 ask for more than any reasonable advocate of the descent 

 theory claims for it. As I have tried to show in the preced- 

 ing pages, the evidence in favor of the theory of descent 

 is not absolutely demonstrative, but the theory is the most 

 satisfactory one that has as yet been advanced to account 

 for the facts. Fleischmann's reference to the common plan 

 of structure of the Gothic churches is not very fortunate for 

 his purpose, since he admits himself that this may be the 

 result of a common tradition handed down from man to man, 

 a sort of continuity that is not very dissimilar in principle from 

 that implied in the descent theory ; in the latter the continuity 

 of substance taking the place of the tradition in the other. 

 Had the plan for each, or even for many of the churches, 

 originated independently in the mind of each architect, then 

 the similarity in style would have to be accounted for by a 



