element must either have possessed the power of spontaneous movement through the water or over damp surfaces, or have been carried by currents of water to the female organs.  That some of the most ancient plants, such as ferns, possessed true sexual organs there can hardly be a doubt; and this shows, as Hildebrand remarks, at how early a period the sexes were separated. (10/43.  ‘Die Geschlechter-Vertheilung’ 1867 pages 84-90.) As soon as plants became phanerogamic and grew on the dry ground, if they were ever to intercross, it would be indispensable that the male fertilising element should be transported by some means through the air; and the wind is the simplest means of transport.  There must also have been a period when winged insects did not exist, and plants would not then have been rendered entomophilous.  Even at a somewhat later period the more specialised orders of the Hymenoptera, Lepidoptera, and Diptera, which are now chiefly concerned with the transport of pollen, did not exist.  Therefore the earliest terrestrial plants known to us, namely, the Coniferae and Cycadiae, no doubt were anemophilous, like the existing species of these same groups.  A vestige of this early state of things is likewise shown by some other groups of plants which are anemophilous, as these on the whole stand lower in the scale than entomophilous species.

There is no great difficulty in understanding how an anemophilous plant might have been rendered entomophilous.  Pollen is a nutritious substance, and would soon have been discovered and devoured by insects; and if any adhered to their bodies it would have been carried from the anthers to the stigma of the same flower, or from one flower to another.  One of the chief characteristics of the pollen of anemophilous plants is its incoherence; but pollen in this state can adhere to the hairy bodies of insects, as we see with some Leguminosae, Ericaceae, and Melastomaceae.  We have, however, better evidence of the possibility of a transition of the above kind in certain plants being now fertilised partly by the wind and partly by insects.  The common rhubarb (Rheum rhaponticum) is so far in an intermediate condition, that I have seen many Diptera sucking the flowers, with much pollen adhering to their bodies; and yet the pollen is so incoherent, that clouds of it are emitted if the plant be gently shaken on a sunny day, some of which could hardly fail to fall on the large stigmas of the neighbouring flowers.  According to Delpino and Hermann Muller, some species of Plantago are in a similar intermediate condition. (10/44.  ’Die Befruchtung’ etc. page 342.)