In this and the two following chapters I shall discuss, as well as the difficulty of the subject permits, the several laws which govern Variability. These may be grouped under the effects of use and disuse, including changed habits and acclimatisation–arrests of development–correlated variation–the cohesion of homologous parts–the variability of multiple parts–compensation of growth–the position of buds with respect to the axis of the plant–and lastly, analogous variation. These several subjects so graduate into each other that their distinction is often arbitrary.

It may be convenient first briefly to discuss that co-ordinating and reparative power which is common, in a higher or lower degree, to all organic beings, and which was formerly designated by physiologists as the nisus formativus.