9 2 SCIENCE PROGRESS 



agent. They are well illustrated by Darwin's discussion of 

 the causes of variability in Chapter XXII. of his Animals and 

 Plants under Domestication, perhaps the least satisfactory that 

 ever came from his pen. In it he finally decides that the 

 variability of organic beings under domestication, "although 

 so general, is not an inevitable contingent on growth and 

 reproduction, but results from the conditions to which the 

 parents have been exposed. Changes of any kind in the con- 

 ditions of life, even extremely slight changes, suffice to cause 

 variability." On the other hand, as to two of the most important 

 changes that can be thought of, " a change of climate is not 

 one of the most potent causes," and " it is doubtful whether 

 a change in the nature of food is a potent cause." On the one 

 hand, " of all the causes which induce variability, excess of food 

 ... is probably the most powerful." On the other hand, " the 

 goose and the turkey have been well fed for many generations, 

 yet have varied very little," while the thorn, hardly cultivated 

 at all, has varied much, and " seeds taken from common English 

 forest trees, grown under their native climate, not highly 

 manured or otherwise artificially treated, yield seedlings which 

 vary much, as may be seen in every extensive seed-bed." 

 Although " we may conclude with certainty that crossing is 

 not necessary for variability," yet " the crossing of distinct 

 species, besides commingling their characters, adds greatly to 

 their variability"; but as against this, "close inter-breeding 

 induces lessened fertility and a weakened constitution ; hence 

 it may lead to variability." If on the one hand a plant varies 

 on being cultivated, the change of conditions is supposed to 

 be the cause, but if, as in the case of the Swan River daisy, 

 no conspicuous change occurs until after seven or eight years 

 of high cultivation, this is taken for "good evidence that the 

 power of changed conditions accumulates." Surely all this is 

 very indifferent logic. It almost amounts to this, that the 

 presence of any one of the suggested causes and its absence 

 are equally effective in inducing variability, if and when it 

 occurs. Moreover, in a creature so shielded from the environ- 

 ment as is the germ-cell of the higher animals, all these influences 

 seem too remote. Even in monads, far more exposed to them, 

 changes of food and environment, except perhaps those occurring 

 at the moment of fission, would presumably rather affect the 

 rate of growth and reproduction of the cell than the manner 



