MENDELIAN LAWS OF INHERITANCE 289 



characters of both parents combined in various degrees. More- 

 over, in many instances, entirely new characters, not seen in 

 either parent, arise among the offspring of succeeding generations 

 of hybrids. 



(v) Hybridisation is usually, though not always, reciprocal : 

 if the pollen of a species A is effective upon the ovules of 

 another species B, the pollen of B is usually similarly effective 

 upon the ovules of A. 



In most instances there is no difference between the offspring 

 of reciprocal crosses. 



It has been noticed also that in the crossing of certain species 

 the hybrids produced always resemble one of the species more 

 than the other, no matter whether it is used as the male or the 

 female parent of the cross. 



Almost all hybrids are more easily crossed with pollen derived 

 from one of the parent species than with pollen from its own 

 flowers or from flowers of another hybrid of the same origin as 

 itself : the progeny of such crossing are termed derivative hybrids. 



Most derivative hybrids are intermediate between the parent 

 and the original hybrid : they are more fruitful than the 

 latter, and some of them frequently come true from seed. 

 If such hybrids are again pollinated by the same parent, 

 the progeny or members of the third generation resemble the 

 pollinating parent more closely ; by a repetition of the crossing 

 with the same parent up to the fourth or fifth generation, all 

 trace of the original second parent of the hybrid is lost or un- 

 recognisable in the progeny. 



True hybrids may be crossed with another species different 

 from either of the parents, and the offspring, which may be 

 termed trispecific hybrids, can be crossed again with still another 

 distinct species. In this manner plants have been obtained 

 combining the characters of three, four, or more species : the 

 offspring of such crossed plants are very variable. 



7. Mendelian laws of inheritance. — (i) Since 1900 much 



T 



