150 EXPERIMENTAL EVOLUTION LECT. 



tions concerning the fauna of the Quercy phosphorites. 

 For instance, he shows that at St. Gerand le Puy, 

 Amphicyon, that presumed ancestor of our dog, has 

 a great tendency to vary in height, strength, form of 

 the head, and in the teeth, while a number of other 

 genera exhibit the same tendency in all the parts of 

 the body : such are Palaeot her turn y EurytJiermm, 

 Acerotheriuni) Palaeocliaerus, Cynodictis, Hyaenodon, 

 Cynohyaenodon, Achirogalus, Necrolemur, Adapts, 

 Psendelurus, etc. ; and so far as dimensions are con- 

 cerned adult fossil remains are often met which 

 indicate that the differences were often from one to 

 two. Nor can all these differences be explained as 

 of mere age or sex. 



As I have already noticed, we know nothing of the 

 causes of variability. We can measure and appreciate 

 it to some extent, but we do not know how it is 

 determined in most cases, save in those where it is 

 consequent upon some modification in external 

 environment, and to these we shall have to revert 

 further on. Must we believe in some innate " tendency 

 towards variation " ? Buffon thought that every species 

 tends towards degeneracy, while Lucas believed in 

 some innate tendency towards the production of new 

 forms, which is, of course, in constant warfare with 

 conservative heredity. Naudin and Naegeli similarly 

 believe in internal forces, and Delbceuf, the eminent 



