IV. GEOLOGICAL PERIODS OF MUTATION. 
12. THE PERIODICITY OF PROGRESSIVE MUTATIONS. 
The essence of the theory of mutation lies within the 
narrow limits 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 be- 
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. 1 In doing so 
I shall confine myself as much as possible to the opinions 
1 See my lecture delivered before the association of German nat- 
uralists and medical men at Hamburg in September 1900, Die Muta- 
tionen vnd die Mutationspenoden bei der Entstehung der Arten 
(Leipsic: Veit & Co., TQOT). 
