VARIABILITY IS AN ADAPTATION 101 



evidence, not that the germ-plasm is directly affected by its 

 surroundings, but that Natural Selection, which controls and 

 which formerly limited the amount of variability, no longer does so 

 to the same extent. 



163. The foregoing leads us to a very important problem. 

 Though environments as a whole are very stable, yet they are 

 seldom if ever absolutely stable in every particular for prolonged 

 periods. One or other factor amongst the complex of which they 

 are compounded tends always to be undergoing change. New 

 factors of elimination appear, or old factors become more stringent 

 or less stringent. Thus, during the last few centuries many new 

 and fatal diseases have appeared amongst the human inhabitants 

 of the Western Hemisphere ; while in the Eastern world the 

 increase of population has accentuated the stringency of selection 

 by old-established diseases at the same time that elimination by 

 war, famine, and wild beasts has diminished. Species change in 

 adaptation to changing environments, and thus all forms of life 

 known to us have arisen. The fact that they have undergone 

 evolution is proof that their environments have undergone change. 

 Man, for example, is not fitted for the same environment as that 

 in which his pre-human ancestors existed, nor would the latter be 

 fitted for his present surroundings. 1 The evolution of a species 

 is founded on the variations of its individuals. Were offspring 

 exact reproductions of parents, the race would persist only if it 

 were well adapted to a perfectly stable environment. A right 

 degree of spontaneous variability therefore is an essential condition 

 of persistence. Presumably, therefore, it is an adaptation with 

 insusceptibility to the direct action of the environment ', the most useful 

 and important of all adaptations ; as much an adaptation as hands, 

 eyes, lungs, or any other of the functioning characters of living 

 types. It is not, as is often implied, a chance property of germ- 

 cells, comparable, for example, to the colour of a dead leaf. And, 

 if it is an adaptation, then, reasoning by analogy, it has, like all 

 other adaptations, been established and maintained by Nattiral 

 Selection. 



164. We need not attempt to discuss at length the question as 

 to how variability began in living beings. Like all other problems 

 relating to the beginnings of life, its origins are involved in 

 obscurity. Variations of some sort must have preceded Natural 

 Selection, if only for a moment, for otherwise the latter could have 

 had no material with which to work. But, probably, even these 



1 See 770. 



