Chap. XII. GENERAL RESULTS. 459 



in the result between a cross with one of the inter- 

 crossed plants and with a fresh stock. These inter- 

 crossed plants tended also in a few cases to become 

 somewhat more uniform in some of their external cha- 

 racters than they were at first. With respect to the 

 plants which were self-fertilised in each generation, 

 their sexual elements apparently lost, after some years, 

 all differentiation, for a cross between them did no 

 more good than a cross between the flowers on the 

 same plant. But it is a still more remarkable fact, that 

 although the seedlings of Mimulus, Ipomoea, Dianthus, 

 and Petunia which were first raised, varied excessively in 

 the colour of their flowers, their offspring, after being 

 self-fertilised and grown under uniform conditions for 

 some generations, bore flowers almost as uniform in 

 tint as those on a natural species. In one case also 

 the plants themselves became remarkably uniform in 

 height. 



The conclusion that the advantages of a cross 

 depend altogether on the differentiation of the sexual 

 elements, harmonises perfectly with the fact that an 

 occasional and slight change in the conditions of life 

 is beneficial to all plants and animals.* But the 

 offspring from a cross between organisms which have 

 been exposed to different conditions, profit in n in- 

 comparably higher degree than do young or old beings 

 from a mere change in their conditions. In this 

 latter case we never see anything like the effect 

 which generally follows from a cross with another 

 individual, especially from a cross with a fresh stock. 

 This might, perhaps, have been expected, for the 

 blending together of the sexual elements of two dif- 

 ferentiated beings will affect the whole constitution at 



* I have given sufficient evi- ation under Domestication,* eh 

 dence on this head in my ' Vuri- xviii. vol. ii. 2nd edit p. 127. 



