ioo VARIABILITY 



the germ-plasm in the different cells is dissimilar (i.e. has varied 

 spontaneously) that the changes, if any, can be dissimilar. But, 

 if it be admitted that germ-cells vary spontaneously, it is un- 

 necessary to appeal to the action of the environment! to explain 

 facts which are already sufficiently explained. In science " neither 

 more nor more onerous causes are to be assumed than are necessary 

 to account for the phenomena." 



162. The apparently well authenticated fact that species (e.g. 

 wild plants) removed to a new environment (e.g. cultivated garden) 

 tend, though healthy and prolific, to display, especially after the 

 lapse of several generations, greater variability than in the ancestral 

 habitats, has also been advanced in support of the hypothesis that 

 variations are commonly caused by the direct action of the en- 

 vironment. Here again we are asked to believe that an influence 

 (i.e. the sum of the new influences in the new environment, which 

 presumably is much the same for all or nearly all the germ-cells) 

 causes variations all round the specific mean. But Natural Selec- 

 tion not only adapts species to changing environments but keeps 

 adapted species stable in stable environments. Speaking generally, 

 natural environments are very stable so stable that all distinct 

 species are very old, and some have persisted without appreciable 

 change for enormous epochs of time. During the entire historical 

 era, though natural varieties may have arisen, not a single species 

 is known to have altered appreciably as a whole. Variability is 

 not lacking, since man is able to create varieties very swiftly by 

 artificial selection. Obviously, any considerable variation in a 

 species, already closely adapted by thousands of years of selection 

 to its environment, is almost sure to be disadvantageous. A 

 superior tendency to vary, itself a variation, is, therefore, disadvan- 

 tageous, and is eliminated in an environment to which the species 

 is already well adapted. But it is advantageous in a new 

 environment to which the species must become adapted, and not 

 unfavourable, or not so unfavourable in a new environment (e.g. 

 garden) to which the species is adapted, but where the old causes 

 of elimination do not act. In brief terms the characters which 

 adapt species to their environments include a right degree of 

 variability ; that is, the degree of variability displayed by every 

 species is itself an adaptation. When monsters, for example, 

 perish, it is not only unfavourable variations which are eliminated, 

 but also a tendency to vary to an extent too great to produce fitness 

 to the parental environment. It follows, then, that the fact that 

 species tend to become more variable in a new environment is 



