CHAPTER 5.  LAWS OF VARIATION.

Effects of external conditions.  Use and disuse, combined with natural selection; organs of flight and of vision.  Acclimatisation.  Correlation of growth.  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 in organic beings under domestication, and in a lesser degree in those in a state of nature—­had been due to chance.  This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation.  Some authors believe it to be as much the function of the reproductive system to produce individual differences, or very slight deviations of structure, as to make the child like its parents.  But the much greater variability, as well as the greater frequency of monstrosities, under domestication or cultivation, than under nature, leads me to believe that deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generations.  I have remarked in the first chapter—­but a long catalogue of facts which cannot be here given would be necessary to show the truth of the remark—­that the reproductive system is eminently susceptible to changes in the conditions of life; and to this system being functionally disturbed in the parents, I chiefly attribute the varying or plastic condition of the offspring.  The male and female sexual elements seem to be affected before that union takes place which is to form a new being.  In the case of “sporting” plants, the bud, which in its earliest condition does not apparently differ essentially from an ovule, is alone affected.  But why, because the reproductive system is disturbed, this or that part should vary more or less, we are profoundly ignorant.  Nevertheless, we can here and there dimly catch a faint ray of light, and we may feel sure that there must be some cause for each deviation of structure, however slight.

How much direct effect difference of climate, food, etc., produces on any being is extremely doubtful.  My impression is, that the effect is extremely small in the case of animals, but perhaps rather more in that of plants.  We may, at least, safely conclude that such influences cannot have produced the many striking and complex co-adaptations of structure between one organic being and another, which we see everywhere throughout nature.  Some little influence may be attributed to climate, food, etc.:  thus,