Chap. IV. 
NATUKAL SELECTION. 
93 
When our plant, by this process of the continued 
preservation or natural selection of more and more 
attractive flowers, had been rendered highly attractive 
to insects, they would, unintentionally on their part, 
regularly carry pollen from flower to flower; and that 
they can most effectually do this, I could easily show 
by many striking instances. I will give only one—not 
as a very striking case, but as likewise illustrating one 
step in the separation of the sexes of plants, presently 
to be alluded to. Some holly-trees bear only male 
flowers, which have four stamens producing a rather 
small quantity of pollen, and a rudimentary pistil; 
other holly-trees bear only female flowers; these have a 
full-sized pistil, and four stamens with shrivelled anthers, 
in which not a grain of pollen can be detected. Having 
found a female tree exactly sixty yards from a male 
tree, I put the stigmas of twenty flowers, taken from 
different branches, under the microscope, and on all, 
without exception, there were pollen-grains, and on 
some a profusion of pollen. As the wind had set for 
several days from the female to the male tree, the 
pollen could not thus have been carried. The weather 
had been cold and boisterous, and therefore not favour¬ 
able to bees, nevertheless every female flower which 
I examined had been effectually fertilised by the bees, 
accidentally dusted with pollen, having flown from tree 
to tree in search of nectar. But to return to our 
imaginary case: as soon as the plant had been ren¬ 
dered so highly attractive to insects that pollen was 
regularly carried from flower to flower, another pro¬ 
cess might commence. No naturalist doubts the advan¬ 
tage of what has been called the ^^physiological divi¬ 
sion of labour;’’ hence we may believe that it would 
be advantageous to a plant to produce stamens alone in 
one flower or on one whole plant, and pistils alone in 
