660 Geological Periods of Mutation. 
day, are extremely rare. 1 On the contrary we usually 
see that the smaller groups sooner or later die out, whilst 
it is only in those cases in which variability, that is to say 
the production of new forms, has been most active that 
the groups continue for longer periods of time. Incapac- 
ity to vary dooms a group to death ; only those who can 
most easily and quickly adapt themselves to changing 
conditions of life can survive. Species-forming variabil- 
ity is therefore not a universal capacity for variation, but 
only the result of quite special conditions which may often 
be absent from certain groups. 
If we assume that the mutability in the main lines of 
the pedigree is an uninterrupted condition, and that this 
power, once lost, cannot be regained, it is clear that every 
branch of the pedigree, i. e., every larger or smaller 
group, is doomed to extinction as soon as the mutable 
species in it become extinct from some cause or another. 
On the other hand it is easy to see that the more numer- 
ous the mutable types are, the greater is the species- 
forming capacity of the whole group and, consequently, 
the greater its prospect of maintenance throughout geo- 
logical ages. 
Without giving a definite expression of opinion, it 
does not seem to me to be likely that mutability has con- 
tinued throughout geological times without interruption. 
Therefore I think it more probable that there has been 
alternation between mutable and immutable periods. This 
latter view, moreover, is in agreement with the conclu- 
sions arrived at by ROSA. 
'Further instances are given by HUXLEY, Proceed. Roy. Inst., 
Ill, p. 151 ; and by POULTON, Brit. Assoc., 1896, Zool. Section, Presi- 
dential Address. For the Foraminifera see CARPENTER, Introduction 
to the Study of the Foraminifera, 1862, p. xi, etc. 
