The Evidence from Variation. 157 



Saltatory Evolution. 



Darwin believes that the evolution of wild species is 

 due, like the formation of many domesticated races, to 

 very slow modification by the natural selection of great 

 numbers of very slight and inconspicuous variations, bufc 

 many other authors have given reasons for believing 

 that this is not the case. 



Many of our most peculiar domestic races have origi- 

 nated suddenly, and there are reasons for believing that 

 the history of the evolution of each species is divided 

 into periods of abrupt and extensive modification, alter- 

 nating with periods of comparative stability. This sub- 

 ject, like those which have been briefly noted in the 

 last two sections, will be fully discussed in Chapter XI., 

 and I will only dwell upon it long enough at present to 

 point out that our view of the cause of variation implies 

 that any particular change should in itself be a fruitful 

 source of still greater modification, so that as soon as a 

 tendency to vary becomes established it will continue to 

 increase until an equilibrium is again established by the 

 natural selection of those modifications which are adapted 

 to the environment. 



Correlated Variation. 



This subject will be fully discussed in the chapter on 

 homology, but a few words upon it will not be out of place 

 here. 



Darwin, who frequently uses the term, includes under 

 it facts which belong to two somewhat different classes. 

 When any part varies, the organs with which it is most 

 directly associated also tend to vary in such a way as to 

 restore the harmonious adjustment between the various 

 parts: and a variation in one part is often accompanied 

 by variation in homologous parts. 



