As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.

Effects of changed conditions -- Use and disuse, combined with natural selection; organs of flight and of vision -- Acclimatisation -- Correlated variation -- Compensation and economy of growth -- False correlations -- Multiple, rudimentary, and lowly organised structures variable -- Parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable -- Species of the same genus vary in an analogous manner -- Reversions to long-lost characters -- Summary.

I have hitherto sometimes spoken as if the variations--so common and multiform with organic beings under domestication, and in a lesser degree with those under nature--were due to chance.