660 Geological Periods of Mutation. 



day, are extremely rare.^ On the contrary we usually 

 see that the smaller groups sooner or later die out, whilst 

 it is only in those cases in which \ariahility, 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 vaiy dooms a group to death ; only those who can 

 most easily and quickly adapt themselves to changing 

 conditions of life can survive. Species-forming varialjil- 

 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 Foraminifcra see Carpenter^ Introduction 

 to the Study of the Foraniinifera, 1862, p. xi, etc. 



