90 PLANT LIFE ON THE FARM. 



On alighting, they are often compelled by the peculiar con- 

 struction and mechanism of the flower to enter or leave it 

 in such a way that they must in the one case brush out the 

 pollen, and in the other deposit it on the stigma. It will 

 be noted how this process of cross-fertilisation favours 

 that process of variation to which allusion has previously 

 been made. 



The farmer wishing to keep his stock of seed turnips 

 pure knows how difficult it is to do this, not only because 

 cross-fertilisation is the rule in the particular variety he 

 wishes to grow, but also because, if any other variety is 

 grown in the neighbourhood, its pollen is sure to impreg- 

 nate his variety and produce a mongrel offspring. Cross- 

 fertilisation then acts in antagonism to the hereditary ten- 

 dency, and secures variation and not only variation, but 

 more vigorous and robust constitution, and more numerous 

 and more healthy descendants. 



Self-fertilisation, or "in and in breeding," occurs, no 

 doubt, in some instances, especially in cereal grasses ; and 

 there are indeed many cases where, for various reasons 

 which need not be cited here, no other mode of fertilisation 

 is possible. Kelative permanence of character is secured 

 by this means, and if constitutional vigour and the health 

 of the offspring be impaired by the long continuance of the 

 process, these ill-effects are provided against by the cir- 

 cumstance that a comparatively trifling change in the 

 flower, or in the circumstances by which it is surrounded, 

 will suffice to prevent self-fertilisation and secure cross- 

 fertilisation. 



Fertilisation of Cereals. It has very generally been 

 stated that the wind is the special agency by means of 



