Theories of Evolution 25 



not rarely open to doubt. Large numbers of 

 hybrid plants and hybrid races are in existence, 

 concerning the origin of which it is impossible 

 to decide. It is impossible in many in- 

 stances to ascertain whether they are of 

 hybrid or of pure origin. Often there is only 

 one way of determining the matter; it is to 

 guess at the probable parents in case of a cross 

 and to repeat the cross. This is a point which 

 always requires great care in the interpretation 

 of unusual facts. 



Three cases are to be distinguished as to 

 heredity. Many plants are so constituted as to 

 be fertilized with their own pollen. In this 

 case the visits of insects have simply to be ex- 

 cluded, which may be done by covering plants 

 with iron gauze or with bags of prepared paper. 

 Sometimes they fertilize themselves without 

 any aid, as for instance, the common evening- 

 primrose; in other cases the pollen has to be 

 placed on the stigma artificially, as with La- 

 marck's evening-primrose and its derivatives. 

 Other plants need cross-fertilization in order to 

 produce a normal yield of seeds. Here two 

 individuals have always to be combined, and 

 the pedigree becomes a more complicated one. 

 Such is the case with the toad-flax, which is 

 nearly sterile with its own pollen. But even in 

 these cases the visits of insects bringing pollen 



