244 EXPERIMENTAL EVOLUTION LECT. 



Observation has shown that crossing is beneficial in 

 some cases only, and many persons have spoken 

 unfavourably of it. This is doubtless due to the fact 

 that we are yet ignorant of some conditions which are 

 of high importance in the matter, and it would seem 

 that facts of apparently little significance have a great 

 deal to do with the ultimate result. Fertility and 

 sterility may be induced by very feeble external 

 agencies, and at the beginning results may be obtained 

 in crossing, which are quite unfavourable, and quite 

 unlike those which are obtained later on. On the other 

 hand some conformity must exist between the plants 

 or animals between whom the crossing occurs. When 

 it does exist and, in fact we hardly know when and 

 where it does or does not the results may be un- 

 favourable in that the characters of the parents, though 

 apparent in the progeny, are not blended together, as 

 was expected ; they are separate, and may be seen side 

 by side instead of being combined. Such is the case, 

 for instance, when white-seeded peas are crossed with 

 green-seeded varieties ; in the progeny green peas arc 

 met amidst white seeds, some seeds are green striped 

 with white ; none are between green and white. 

 Similar instances are met with among animals when 

 among the progeny of the same parents, some exhibit 

 the character of the one, and others those of the other 

 parent. Another point which requires consideration 



