BOTANICAL EVOLUTION, 461 
generations. After pretty close investigation of the subject I am 
strongly inclined to believe that this is a law of nature throughout 
the vegetabie and animal kingdoms.” And we now know from the 
experiments of an immense number of workers, and from the 
extraordinary results which have followed from such experiments that 
cross-fertilization is extremely beneficial to plants, and that even those 
plants which appear to us to be constantly self-fertilized, are probably 
every now and then subjected to a cross, by which the vitality of the 
stock is increased. When, then, insects took to visiting flowers for 
their pollen, they would tend to cross-fertilize them, and in this way 
some flowers and plants would be benefited more than others. The 
most minute variation which might tend to attract insects to the 
flower, would also tend—by the law enuntiated before—to be 
reproduced and intensified, and thus might arise shades of colour 
in the calyx and bracts, as well as the production of saccharine 
secretions or nectar. At the same time, the outer stamens would 
gradually become differentiated or specialized into conspicuous leaves 
tending to retain the yellow colour and giving up their function of 
producing pollen. Also in some cases along with this development 
of petals serving for attraction only, and of the nectar, would 
probably arise an intensification of the odour. Probably all flowers, 
whether scentless to us or not, exercise some influence on the 
olfactory nerves of insects, and wherever any variation in this scent 
tended to make it attractive to suitable insects, that variation would 
be increased and developed. Now it will help to show you how 
difficult it is to follow out any such line of development, when I point 
out to you another line of development which advanced along with it, 
and modified the first. We say that flowers developed their bright 
petals, their scent and their nectar for the sake of attracting insects 
to fertilize them. But of course at the same time while doing so they 
became equally attractive to insects which did not fertilize them, and 
which therefore we must consider as noxious to them, sceing they 
carried off their nectar and pollen without giving any service in 
compensation. So that another cause of variation constantly giving 
rise to new developments of structure originated in this attempt to 
keep off these unbidden guests. I cannot possibly within the limits 
of one lecture follow out this line of evolution, because the devices are 
innumerable, and the whole question is so inextricably mixed up that 
we cannot take time, even if we had the means to weigh all the causes 
which have been at work. But I may just point out that creeping 
insects of all kinds are unable to carry pollen from plant to plant, and 
therefore being unsuitable, are kept off in a variety of ways. Thus 
ants alone are a class of flower robbers, and they are prevented from 
getting to some flowers by the hair on the stems (Boraginece), by 
viscid secretions formed on glandular hairs (Petunia), by wells of 
water formed by connate leaves (Lonicera), by thin epidermis covering 
laticiferous vessels (Lettuce) and in one or two remarkable cases 
mentioned by Belt (‘‘Naturalist in Nicaragua”) by an odd device. 
Thus, a certain species of Acacia termed the Bull’s-horn thorn is 
provided with remarkable hollow thorns which serve as domiciles for 
& species of small fierce ant. “These ants form a most efficient 
standing army for the plant, which prevents not only mammalia from 
browsing on the leaves, but delivers it from the attacks of a much 
'more dangerous enemy—the leaf-cutting ants. For these services 
