62 EVOLUTION AND NATURAL THEOLOGY. 



which has once received such an impetus is 

 liable to vary more and more, till it again 

 settles into a state of comparative stability. 

 He instanced the imperceptible formation of the 

 Europeo-American race as an illustration of 

 the manner in which new species or races 

 may arise before our very eyes, without our 

 being able to distinguish the steps by which 

 the modification was brought about.* 



It can scarcely be doubted that both gradual 

 and sudden variations have acted in the pro- 

 duction of the system of nature as we now see 

 it. In cases analogous to that of the domestic 

 pigeon, where there is ab initio a strong tendency 

 to variation, it is probable that gradual 

 modification may have sufficed in time for the 

 differentiation of species ; but in cases similar 

 to that of the peacock, a bird which varies 

 very little under ordinary circumstances, it is 

 more likely that variation, if it occurs at all, 

 will occur suddenly, like a " sport " among 

 plants. 



* Our common Small White Butterfly (Pieris Rapes) has lately been 

 introduced into Canada and the United States, where it has completely 

 established itself. An extraordinary yellow variety almost unknown 

 in Europe, is of frequent occurrence in America; and it is quite possible 

 that this may ultimately become the ordinary American form. 



