vii OX THE INFERTILITY OF CROSSES 155 



to note that they occur also in the vegetable kingdom. 

 Allowing for all the circumstances which are known to 

 prevent the production of seed, such as too great luxuriance 

 of foliage, too little or too much heat, or the absence of 

 insects to cross-fertilise the flowers, Mr. Darwin shows that 

 many species which grow and flower with us, apparently in 

 perfect health, yet never produce seed. Other plants are 

 affected by very slight changes of conditions, producing seed 

 freely in one soil and not in another, though apparently 

 growing equally well in both ; while, in some cases, a 

 difference of position even in the same garden produces a 

 similar result. 1 



Reciprocal Crosses. 



Another indication of the extreme delicacy of the 

 adjustment between the sexes, which is necessary to produce 

 fertility, is afforded by the behaviour of many species and 

 varieties when reciprocally crossed. This will be best 

 illustrated by a few of the examples furnished us by Mr. 

 Darwin. The two distinct species of plants, Mirabilis jalapa 

 and M. longiflora, can be easily crossed, and will produce 

 healthy and fertile hybrids when the pollen of the latter is 

 applied to the stigma of the former plant. But the same 

 experimenter, Kolreuter, tried in vain, more than two hundred 

 times during eight years, to cross them by applying the pollen 

 of M. jalapa to the stigma of M. longiflora. In other cases two 

 plants are so closely allied that some botanists class them as 

 varieties (as with Matthiola annua and M. glabra), and yet 

 there is the same great difference in the result when they are 

 reciprocally crossed. 



Individual Differences in respect to Cross-Fertilisation, 



A still more remarkable illustration of the delicate 

 balance of organisation needful for reproduction, is afforded 

 by the individual differences of animals and plants, as regards 

 both their power of intercrossing with other individuals or 

 other species, and the fertility of the offspring thus produced. 

 Among domestic animals, Darwin states that it is by no means 

 rare to find certain males and females which will not breed 



1 Darwin's Animals and Plants under Domestication, vol. ii. pp. 163-170. 



