116 CONCLUDING EEMAEKS. Chap. XXVIIL 



intervals of time, and likewise in the several striking cases 

 given of analogous or parallel variations. Although some 

 of these latter cases are due to reversion, others cannot thus 

 be accounted for. 



From the indirect action of changed conditions on the 

 organisation, owing to the reproductive organs being thus 

 affected — from the direct action of such conditions, and these 

 will cause the individuals of the same species either to vary 

 in the same manner, or differently in accordance with slight 

 differences in their constitution — from the effects of the in- 

 creased or decreased use of parts — and from correlation, — 

 the variability of our domesticated productions is complicated 

 to an extreme degree. The whole organisation becomes 

 slightly plastic. Although each modification must have its 

 own exciting cause, and though each is subjected to law, 

 yet we can so rarely trace the precise relation between cause 

 and effect, that we are tempted to speak of variations as if 

 they arose spontaneously. We may even call them accidental, 

 but this must be only in the sense in which we say that 

 a fragment of rock dropped from a height owes its shape to 

 accident. 



It may be worth while briefly to consider the result of the 

 exposure to unnatural conditions of a large number of animals 

 of the same species and allowed to cross freely with no 

 selection of any kind, and afterwards to consider the result 

 when selection is brought into play. Let us suppose that 

 500 wild rock-pigeons were confined in their native land in 

 an aviary and fed in the same manner as pigeons usually 

 are ; and that they were not allowed to increase in number. 

 As pigeons propagate so rapidly, I suppose that a thousand 

 or fifteen hundred birds would have to be annually killed. 

 After several generations had been thus reared, we may 

 feel sure that some of the young birds would vary, and 

 the variations would tend to be inherited: for at the 

 present day slight deviations of structure often occur and 

 are inherited. It would be tedious even to enumerate the 

 multitude of points which still go on varying or have 

 recently varied. Many variations would occur in correla- 



