Page:The Effect of External Influences upon Development.djvu/55

Rh finely graduated and regulated the sensitiveness of parts may be in the case of the gradual differentiation of the body in plants and animals according to the principle of division of labour; for sensitiveness has gone hand in hand with morphological differentiation, gradually becoming more finely graduated and increased. And how wonderfully fine it can become we see by reference to the various special activities of the elements of our own body, as well as to the numerous processes of all kinds which occur both in plants and animals. For example, we know that as a rule self-fertilization is avoided among Phanerogams, that numerous complicated arrangements exist in the flowers to prevent its occurrence, and that even the very existence of many flowers depends on the fact that cross-fertilization by the agency of insects is an advantage. This presupposes that self-fertilization is always possible, and that it would always be less advantageous in these cases. But neither of these suppositions is universally true:—all degrees of sensitiveness to self-pollination exist. Many orchids which are in every respect adapted for cross-fertilization by insects, have proved to be fully fertile with their own pollen; others, on the other hand, like Corydalis cava, which could very easily effect self-pollination, are quite unfruitful with their own pollen: in contrast to these, again, we have cleistogamic flowers, which never open, and consequently are designed for self-fertilization only. Most striking of all, however, is the varying sensitiveness for minute differences of stimulus in the case of dimorphic and trimorphic flowers, which are so