to come to this conclusion from finding, as we shall hereafter show, that changes of the natural conditions of existence seem peculiarly to affect the action of the reproductive system{214}. As we see that hybrids and mongrels, after the first generation, are apt to vary much, we may at least conclude that variability does not altogether depend on excess of food. {210} See _Origin_, Ed. i. p. 7, vi. p. 7. {211} <Note in the original.> "Isidore G. St Hilaire insists that breeding in captivity essential element. Schleiden on alkalies. <See _Var. under Dom._, Ed. ii. vol. II. p. 244, note 10.> What is it in domestication which causes variation?" {212} <Note in the original.> "It appears that slight changes of condition <are> good for health; that more change affects the generative system, so that variation results in the offspring; that still more change checks or destroys fertility not of the offspring." Compare the _Origin_, Ed. i. p. 9, vi. p. 11. What the meaning of "not of the offspring" may be is not clear. {213} In the _Origin_, Ed. i. p. 41, vi. p. 46 the question is differently treated; it is pointed out that a large stock of individuals gives a better chance of available variations occurring. Darwin quotes from Marshall that sheep in small lots can never be improved. This comes from Marshall's _Review of the Reports to the Board of Agriculture_, 1808, p. 406. In this Essay the name Marshall occurs in the margin. Probably this refers to _loc. cit._ p. 200, where unshepherded sheep in many parts of England are said to be similar owing to mixed breeding not being avoided. {214} See _Origin_, Ed. i. p. 8, vi. p. 8.