396 SCOTT. [Vol. V. 



in the way of the addition of new parts or the reduction of 

 those already present, acts just as if the direct action of the 

 environment and the habits of the animal were the efficient 

 cause of the change, and any explanation which excludes the 

 direct action of such agencies is confronted by the difficulty of 

 an immense number of the most striking coincidences. We 

 must all heartily agree with Weismann in saying that " we 

 require, not a mere formal explanation of the origin of species 

 of the most comfortable nature, but the real and rightful expla- 

 nation." Whether his theories, profound and brilliant as they 

 are, are to give us this real and rightful explanation, the future 

 must decide. 



Geological Museum, 



Princeton, N.J., July 20, 1891. 



P.S. — The preceding paper was all in type before I received 

 Lloyd Morgan's very valuable and suggestive article on " The 

 Nature and Origin of Variations " (Proceedings of the Bristol 

 Naturalists' Society, Vol. VI., Part III.). I regret this the more 

 because Professor Morgan seems to have somewhat misappre- 

 hended the character of the evidence to be derived from palae- 

 ontology in favor of the dynamical view of evolution. " Those 

 American zoologists who have studied the evolution of mamma- 

 lian teeth contend that the occurrence of new points and cusps 

 is not indeterminate, but determinate. New elements of tooth- 

 structure appear in definite positions. There is nothing to indi- 

 cate selection from among indefinite variations. Other evidence 

 of like implication is adduced by American evolutionists ; but 

 this of tooth-structure is put forward as the strongest case. I 

 am inclined to think that the facts of palaeontology, so far as 

 they go, point in this direction. But I question whether they 

 can be regarded as conclusive. In criticising the position, stress 

 may be laid on the imperfection of the geological record ; and it 

 may be urged that the number of individuals in our palaeonto- 

 logical collections is not sufficient to constitute a truly represen- 

 tative sample. Furthermore, on the hypothesis of selection, the 

 individuals possessed of teeth with points and cusps in other 

 than these adaptive positions must have been weeded out in the 

 early stages of life. In view of these objections the evidence 



