Andrew Knight formerly maintained that under these circumstances they always become weakly, but this conclusion has been warmly disputed by others.  A recent and competent judge, Professor Asa Gray, leans to the side of Andrew Knight, which seems to me, from such evidence as I have been able to collect, the more probable view, notwithstanding many opposed facts. (12/4.  ’Darwiniana:  Essays and Reviews pertaining to Darwinism’ 1876 page 338.)

The means for favouring cross-fertilisation and preventing self-fertilisation, or conversely for favouring self-fertilisation and preventing to a certain extent cross-fertilisation, are wonderfully diversified; and it is remarkable that these differ widely in closely allied plants,—­in the species of the same genus, and sometimes in the individuals of the same species. (12/5.  Hildebrand has insisted strongly to this effect in his valuable observations on the fertilisation of the Gramineae:  ‘Monatsbericht K. Akad.  Berlin’ October 1872 page 763.) It is not rare to find hermaphrodite plants and others with separated sexes within the same genus; and it is common to find some of the species dichogamous and others maturing their sexual elements simultaneously.  The dichogamous genus Saxifraga contains proterandrous and proterogynous species. (12/6.  Dr. Engler ‘Botanische Zeitung’ 1868 page 833.) Several genera include both heterostyled (dimorphic or trimorphic forms) and homostyled species.  Ophrys offers a remarkable instance of one species having its structure manifestly adapted for self-fertilisation, and other species as manifestly adapted for cross-fertilisation.  Some con-generic species are quite sterile and others quite fertile with their own pollen.  From these several causes we often find within the same genus species which do not produce seeds, while others produce an abundance, when insects are excluded.  Some species bear cleistogene flowers which cannot be crossed, as well as perfect flowers, whilst others in the same genus never produce cleistogene flowers.  Some species exist under two forms, the one bearing conspicuous flowers adapted for cross-fertilisation, the other bearing inconspicuous flowers adapted for self-fertilisation, whilst other species in the same genus present only a single form.  Even with the individuals of the same species, the degree of self-sterility varies greatly, as in Reseda.  With polygamous plants, the distribution of the sexes differs in the individuals of the same species.  The relative period at which the sexual elements in the same flower are mature, differs in the varieties of Pelargonium; and Carriere gives several cases, showing that the period varies according to the temperature to which the plants are exposed. (12/7.  ‘Des Varieties’ 1865 page 30.)