[Illustration: FIG. 50. Flowers of the Willow (_Salix cinerea_); after H. Müller. _A_, the male. _B_, the female catkin. _C_, individual male flower; _n_, nectary. _D_, individual female flower; _n_, nectary. _E_, Poplar, an exceptional hermaphrodite flower.] It is plain that the utilization of the insect-visitors in fertilization might be secured in either of two ways; the female blossoms might also offer something attractive to the insects, or hermaphrodite flowers might be formed. As a matter of fact, both ways have been followed by Nature. An example of the former is the willow, the cross-fertilization of which was forced upon the insects by the development in both female and male blossoms of a nectary (Fig. 50, _C_ and _D_), a little pit or basin in which nectar was secreted. The insects flew now to male and now to female willow-catkins, and in doing so they carried to the stigma of the female blossom the pollen, which in this case was not dusty but sticky, so that it readily adhered to their bodies. The securing of cross-fertilization by the development of hermaphrodite flowers has, however, occurred much more frequently, and we can understand that this method secured the advantageous crossing much more perfectly, for the pollen had necessarily to be carried from blossom to blossom, while, in cases like that of the willow, countless male blossoms might be visited for nectar one after the other before the insect made up its mind to fly to a female blossom of the same species. The beginnings of the modification of the unisexual flowers in this direction may be seen in variations which occur even now, for we not infrequently find, in a male catkin, individual blossoms, which, in addition to the stamens, possess also a pistil with a stigma. (Fig. 50 _E_ shows such an abnormal hermaphrodite flower from a poplar.) As soon as hermaphrodite flowers came into existence the struggle to attract insects began in a more intense degree. Every little improvement in this direction would form the starting-point of a process of selection, and would be carried on and increased to the highest possible pitch of perfection.