VII 
ON 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. longiHora, 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. longiHora. 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. 
