I believe that the conditions of life, from their action on the reproductive system, are so far of the highest importance as causing variability. I do not believe that variability is an inherent and necessary contingency, under all circumstances, with all organic beings, as some authors have thought. The effects of variability are modified by various degrees of inheritance and of reversion. Variability is governed by many unknown laws, more especially by that of correlation of growth. Something may be attributed to the direct action of the conditions of life. Something must be attributed to use and disuse.Some of this passage, the conclusion to the chapter on 'Variation under Domestication', confuses me. I get the impression that what I understand by variation is a more general meaning than that used in this passage.
I also do not know quite what he means by "correlation of growth".
I assume that by "conditions of life" he is talking about deaths by natural causes, and the like.
The comment on "use and disuse" sounds rather Lamarckian. I wonder if it is a throw-away comment, a nod to Lamarck's work.