572 THE FERTILISATLON OF FLOWERS. [PART 1Y, 
surpass all other insects in length of tongue, visit flowers whose 
honey is accessible to them alone. 
We find that the above-mentioned insects often resort, to a 
certain small extent, to flowers less productive than those that 
they usually frequent. It is rare for a plant which produces 
numerous flowers and much honey or pollen to continue so 
abundant for the whole period that an insect is on the wing that 
that insect can confine its visits to it alone. In such cases it is 
probable that the acquired habit is transmitted instinctively from 
one generation to another. 
The study of particular species of insects confirms the con- 
clusion based on observation of the more conspicuous flowers, 
that in general anthophilous insects are not confined by hereditary 
instinct to certain flowers, but fly about seeking their food on 
whatever flowers they can find it. And hence the circumstance, 
already insisted on, that a flower receives the more visits the more 
conspicuous it is, becomes readily intelligible. It is also clear how 
natural selection must have seized upon and perpetuated all 
characters tending to greater conspicuousness. And thus we may 
explain all such phenomena as the development of various colours 
in flowers, the enlargement and increased prominence of the 
coloured surfaces, the association of many flowers together, the 
division of labour in such associations between reproductive and 
merely attractive members of the community, the appearance of 
the flowers before the leaves in Salix, Cornus mascula, ete., and 
many other similar phenomena. 
Effect of Odour. 
The effect of various perfumes in attracting insects is as manifest 
as the effect of conspicuousness ; and observation shows that strong 
scent is an even more powerful lure than bright colours. The 
richly scented flowers of Convolvulus arvensis are far more 
abundantly visited than the larger and more conspicuous but 
scentless flowers of C. sepiwm; the sweet-scented Violet is much 
more visited than the large, brightly coloured, but scentless 
Pansy; the small, insignificant, but strongly perfumed flowers of 
Lepidium sativum surpass in the abundance of their visitors the 
other more conspicuous but scentless Crucifers, 
B.—The Effect of Food Supply. 
Every plant supplies in its pollen a nitrogenous food which is 
readily made use of by many insects, and when once found leads 
