136 Permanence and Evolution. 



to estimate, have been brought forward to show 

 that the time occupied by the past history of 

 organic life, though vast, is still confined within 

 definite and ascertainable limits. But whether 

 this be so or not, I think it is not remembered 

 that when we speak of natural selection gradually 

 by insensible modification producing characters 

 advantageous to the race under particular cir- 

 cumstances, we ought to consider the likelihood 

 that before the organism is vastly changed the 

 circumstances may be changed too, whether the 

 circumstances be cold, heat, moisture or the 

 presence of noxious, or food-producing other 

 organic forms, etc. It will be a race between 

 the permanence of a given type and that of its 

 surroundings, and when we consider that within 

 the period since we can be sure of the existence of 

 the present varieties of man (the polished stone 

 period, probably about 8,000 or 10,000 years ago 

 at least), no organic form seems as far as we 

 know to have changed in a state of nature, while 



