NATURAL SELECTION 



altered, and if each race of plants and animals 

 were always to remain confined to one limited 

 area, the survival of the fittest would simply 

 maintain unaltered any given aspect of the be- 

 ings constituting the organic world. All vari- 

 ations on either side of the well-adjusted mean 

 would be incessantly cut off by natural selection, 

 and species would be immutable. It is need- 

 less to say that no such state of things has ever 

 existed. Constant change has been the order 

 of things ever since our planet first became fit 

 to support organic life. No part of the earth*s 

 surface is now, or ever has been, at rest. Con- 

 tinents are rising and sinking, seas are growing 

 deeper and shallower, soils are constantly alter- 

 ing in chemical composition, rivers are ever 

 changing their beds, solar radiance is ever gain- 

 ing or losing in intensity, according to the 

 earth's ever-varying position in space, the den- 

 sity and moisture of the air are continually in- 

 creasing and diminishing, and every species of 

 plant and animal is continually pressing upon 

 the limits of the area within which it is confined. 

 All these changes are going on to-day, and have 

 been going on during millions of ages. Though 

 so slight as to be recognized only by the most 

 careful observation during the period covered 

 by human history, these changes have during 

 longer periods sufficed to submerge every con- 

 tinent and perhaps to make dry land of every 



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