IV. GEOLOGICAL PERIODS OF MU'l'A'i 1( JX. 



§ 12. THE PERIODICITY OF PROGRESSIVE MUTATIONS. 



The essence of the theory of mutation he.s within the 

 narrow hmits of the Linnean collective species and agrees 

 equally well with the theory of descent with modification 

 and with the doctrine of creation. Its special province 

 is the question how those smaller species originate which 

 were supposed in pre-Darwinian times to have arisen 

 by natural laws from the created types, i. e., from the 

 collective species. 



But the light shed by the new theory extends far \k- 

 yond these narrow limits. Its full importance can better 

 be estimated from a general point of view than by a 

 reconsideration of the facts already given, and the final 

 judgment will probably depend in a larger measure on 

 its applicability to the broad questions of descent, than 

 on the significance of the facts upon which it is based. 



Therefore it seems desirable to show that the muta- 

 tion theory is really in closer accord with present views 

 regarding the phylogeny of plants and animals, in many 

 and indeed in the most important points, than the pre- 

 valent form of the theory of selection.^ Tn doing so 

 I shall confine myself as much as possible to the oj)inions 



* See my lecture delivered before the association of Gernian nat- 

 uralists and medical men at Hamburg in September igoo. Pi> ^futa- 

 tinncn und die Mutafionsf^cnodrn hci dcr Rntstchuttii dcr Artcn 

 (Leipsic: Veit & Co., tqotV 



