Each of these lists contains by a mere accident the same number of genera, namely, forty-nine.  The genera in the first list include sixty-five species, and those in the second sixty species; the Orchideae in both being excluded.  If the genera in this latter order, as well as in the Asclepiadae and Apocynaceae, had been included, the number of species which are sterile if insects are excluded would have been greatly increased; but the lists are confined to species which were actually experimented on.  The results can be considered as only approximately accurate, for fertility is so variable a character, that each species ought to have been tried many times.  The above number of species, namely, 125, is as nothing to the host of living plants; but the mere fact of more than half of them being sterile within the specified degree, when insects are excluded, is a striking one; for whenever pollen has to be carried from the anthers to the stigma in order to ensure full fertility, there is at least a good chance of cross-fertilisation.  I do not, however, believe that if all known plants were tried in the same manner, half would be found to be sterile within the specified limits; for many flowers were selected for experiment which presented some remarkable structure; and such flowers often require insect-aid.  Thus out of the forty-nine genera in the first list, about thirty-two have flowers which are asymmetrical or present some remarkable peculiarity; whilst in the second list, including species which are fully or moderately fertile when insects were excluded, only about twenty-one out of the forty-nine are asymmetrical or present any remarkable peculiarity.

Means of cross-fertilisation.

The most important of all the means by which pollen is carried from the anthers to the stigma of the same flower, or from flower to flower, are insects, belonging to the orders of Hymenoptera, Lepidoptera, and Diptera; and in some parts of the world, birds. (10/1.  I will here give all the cases known to me of birds fertilising flowers.  In South Brazil, humming-birds certainly fertilise the various species of Abutilon, which are sterile without their aid (Fritz Muller ’Jenaische Zeitschrift f.  Naturwiss.’  B. 7 1872 page 24.) Long-beaked humming-birds visit the flowers of Brugmansia, whilst some of the short-beaked species often penetrate its large corolla in order to obtain the nectar in an illegitimate manner, in the same manner as do bees in all parts of the world.  It appears, indeed, that the beaks of humming-birds are specially adapted to the various kinds of flowers which they visit:  on the Cordillera they suck the Salviae, and lacerate the flowers of the Tacsoniae; in Nicaragua, Mr. Belt saw them sucking the flowers of Marcgravia and Erythina, and thus they carried pollen from flower to flower.  In North America they are said to frequent the flowers of Impatiens:  (Gould ‘Introduction to the Trochilidae’