58 CROSS- AND SELF-FERTILISATION 
than is absolutely necessary for fertilisation need be pro- 
duced. If, however, the flower is to be crossed, extra 
expenditure must be made. If the carrying agent for 
pollen be wind or water, a vast amount of pollen must be 
wasted ; if it be insects, though there is less waste, there-' 
must be brightly coloured organs, scent, honey, &c., to 
attract them. Thus it is that the plant has to pay a certain 
price for its cross-fertilisation (this price being affected by 
many factors, as we shall afterwards see), and only when the 
gain resulting from crossing is greater than the price to be 
paid for it will cross-fertilisation prove an advantage to the 
plant 1 . 
The advantages of fertilisation seem to rest upon the 
fact of the two parents having grown under slightly different 
conditions of life. The effect of a cross between two separate 
flowers on a plant A is but little if any better than that of 
the purest self-fertilisation within one flower, and the good 
effect of a cross A x B is greater if A and B have been grown 
at a distance apart than if grown near together. This is 
expressed in Nageli’s law that “ the consequences of fertili- 
sation reach their optimum when a certain mean difference 
in the origin of the sexual cells is attained.” Varieties are 
frequently even more fertile together than plants of the same 
form, but when we go further and cross different species the 
value of fertilisation decreases again, sterility becoming a 
more or less marked feature in such unions. 
Pollination. In all the higher flowering plants there 
must be, before fertilisation can take place, a preliminary 
operation — pollination — consisting in the transport of the 
pollen-grains to the ovules- (Gymnospermae) or to the 
carpels (Angiospermae) : a few general principles of this 
subject call for consideration here, and details are given 
below 2 . 
The general agents , external to the flower, effecting pol- 
lination are wind, animals, and water. The flower may 
1 MacLeod in Bot. Jaarboek , Gent, v. 1893 (review by Willis in 
Science- Progress, Nov. 1895); Muller’s Fertilisation of Flowers \ Sach’s 
History of Botany', F. Darwin in Nature, Oct., 1898, p. 630. 
2 And see Darwin’s Cross - and Self- Fertilisation, Fertilisation of 
Orchids, Forms of Flowers ; Muller, Fertilisation of Flowers, Alpen- 
blumen', MacLeod in Bot. Jaarb. v. 1893 (Dutch); Knuth, Handbuch 
der Bliitenbiologie , &c. 
