No doubt each slight variation must have its efficient cause; but it is as hopeless an attempt to discover the cause of each, as to say why a chill or a poison affects one man differently from another. Even with modifications resulting from the definite action of the conditions of life, when all or nearly all the individuals, which have been similarly exposed, are similarly affected, we can rarely see the precise relation between cause and effect. In the next chapter it will be shown that the increased use or disuse of various organs produces an inherited effect. It will further be seen that certain variations are bound together by correlation as well as by other laws. Beyond this we cannot at present explain either the causes or nature of the variability of organic beings. CHAPTER 2.XXIV. LAWS OF VARIATION--USE AND DISUSE, ETC. NISUS FORMATIVUS, OR THE CO-ORDINATING POWER OF THE ORGANISATION. ON THE EFFECTS OF THE INCREASED USE AND DISUSE OF ORGANS. CHANGED HABITS OF LIFE. ACCLIMATISATION WITH ANIMALS AND PLANTS. VARIOUS METHODS BY WHICH THIS CAN BE EFFECTED. ARRESTS OF DEVELOPMENT. RUDIMENTARY ORGANS. 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--arrest 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 one another that their distinction is often arbitrary. It may be convenient first briefly to discuss that coordinating and reparative power which is common, in a higher or lower degree, to all organic beings, and which was formerly designated by physiologists as nisus formativus.