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 
