VARIATION. 



Darwin in his theory of evolution started from the obser- 

 vation that cdl organisms are seen to vary in every character, 

 in every proportion, every function which is studied. Taking 

 for granted this variabiUty, Darwin reasoned, that such small 

 differences as always exist between members of a single group, 

 must make these individuals better or less-fitted for the con- 

 ditions under which they have to live. 



And as it is manifestly impossible, for all the descendants of 

 all the individuals to find a place on earth, a portion only can 

 survice in each generation. Following this reasoning, and com- 

 paring the situation of species in nature with that of species of 

 domestic animals and plants, where selection brings about a 

 change of type and a reduction of variability, it seemed to 

 Darwin, that the survivors would always be those individuals, 

 which happened to be best adapted to their surroundings. If we 

 assume with Darwin, that those small differences, which we 

 can always observe as existing between the individuals of 

 every group, are hereditable, we can see how a continuance of 

 these two processes, on the one hand a variation, and on the 

 other hand a natural selection which tends to limit and direct 

 this variability, must result in a change. The group affected 

 must become more ana more fit, and the direction of the grad- 

 ual change is given by the conditions under which the group 

 lives. 



Now this is all very dear and rather obvious, but two great 

 difficulties remain. Darwin's principle of natural selection 

 explains very nicely how a species, given a certain variability, 

 can develop some useful quality, and can become pure for this 

 quality, but it rather impUes, that every quality for which a 



