6 10 THE BEGINNINGS OF LIFE. 



been exposed to great changes of climate and have 

 migrated over great distances 1 , whereas in Egypt during 

 the last 3000 years the conditions of life, as far as we 

 know, have remained absolutely uniform/ Mr. Darwin 

 then adds: c The fact of little or no modification 

 having been effected since the glacial period would be 

 of some avail against those who believe in an innate 

 and necessary law of development, but is powerless 

 against the doctrine of Natural Selection or survival of 

 the fittest, which implies only that variations or indi- 

 vidual differences of a favourable nature occasionally 

 arise in a few species, and are then preserved.' The 

 facts above cited are, however, not at all inconsistent 

 with a belief in progressive development, when this 

 belief is limited in the way I have mentioned. And 

 there are, moreover, very excellent reasons for be- 

 lieving that the structures of higher organisms are much 

 less easily modified than those of lower organisms. Mr. 

 Spencer sums up the results of his interesting specula- 

 tions on this subject in the following words 2 : c With- 

 out assuming fixity of species, we find good reason for 

 anticipating that kind and degree of stability which is 

 observed. We find grounds for concluding a priori that 

 an adaptive change of structure will soon reach a point 

 beyond which further adaptation will be slow ; for con- 

 cluding that when the modifying cause has been but 



1 Mr. Spencer would perhaps suggest that their migration may have 

 been one of the means of protecting them from such changes of climate. 



2 ' Principles of Biology,' vol. i. p. 199. 



