Page:The Effect of External Influences upon Development.djvu/56

52 adjusted that self-fertilization never occurs, and only the pollen from another flower of different form can successfully effect fertilization. Here, again, we seem to have an insight into the means by which this adaptation has been brought about: the pollen-grains are of different diameters, and the length of the pollen-tubes to which they give rise corresponds to that of the style of another form of flower. In most other cases we are as yet unable to observe the mechanism of sensitiveness, which is usually of a much finer kind. But a mechanism it must always be, and it can only have originated on the one principle with which we are acquainted in connexion with the origin of adaptations,—viz. by selection, based on individual variation.

Many more facts could be brought forward to illustrate the subject of the present lecture, but I fear that the numerous details I have been obliged to give may have already exhausted your patience, and lessened your interest in the general conclusions to which they lead. I will therefore conclude with a brief summary of my remarks.

It has long been recognized that external influences serve to stimulate the functions of the body, and I have attempted to show that in a great number of cases they also act in another and less apparent manner. They are used—so to speak—by nature to regulate in a purposeful manner the appearance of the various forms which members of a species may take. The germ must thus contain all the primary constituents (Anlagen) of these different forms; and a stimulus-produced by the kind of food, by light, by warmth, or by some other