584 THE GENESIS OF NEW SPECIES. 



The fact that many more than a hundred Willow hybrids are known looks as if the 

 agency of insects were the more favourable to hybridization. At the same time we 

 must bear in mind that the number of species of Willow in Northern and Central 

 Europe is very large, whilst the number of different Birches, Alders, and Oaks is 

 small. Taken in relation to the number of species, the hybrids belonging to the 

 plants last mentioned are no less numerous than those of Willows, and it appears, 

 therefore, that hybrids occur as frequently amongst plants whose pollen is dispersed 

 in the form of dust as amongst plants with adhesive pollen which is transported by 

 insects. The prevalence of hybrids amongst Pond- weeds points to the same conclu- 

 sion. A comparison between the Docks and Polygonums even indicates that in 

 anemophilous plants, such as the Docks, hybrids come into existence more readily 

 than in the case of entomophilous plants as represented by the Polygonums, for in 

 scarcely any genus is the number of hybrids so great in proportion to the number 

 of species as in the Dock genus, and the ratio is certainly higher than it is with the 

 Pol3^gonums. 



As regards the Caryophyllacese it is remarkable that Dianthus has many hybrids 

 and Silene few, although these two genera resemble one another in the distribution 

 of their sexes and in being chiefly visited by lepidopterous insects. In the genus 

 Viola hybrids are extremely common. It has been shown that many Violas which 

 were formerly designated as " transitional forms " are in reality hybrids, and thus 

 the grounds upon which systematic Botanists treated whole series of species as one 

 only are removed. As with the Violas in Central Europe so also with their allies 

 the Cistuses in the South, hybrids are numerous; several kinds of Cistus hybrid occur 

 so commonly that they were described as species by the Botanists of former times. 



It is noticeable in the Cruciferse that no hybrids are formed in nature between 

 the numerous annual species of this family. There are also only a few hybrids 

 known which are derived from the perennial species. The genera Roripa and 

 Draha are, however, exceptional in this respect. The case of Ranunculace^e is 

 similar. In the comprehensive genera Aconitum, Helleborus, and Ranunculus, 

 only a few hybrids have been identified with certainty, whilst in Anemone and 

 Pulsatilla there are almost as many hybrids as species. The case of the hybrid 

 Water-lily Nuphar intermedium, will be discussed later on. 



Many hybrids are known in the genera Tilia, Hypericum,, Malva, Rhamnus, 

 Pistacia, Acer, Euphorbia, and Epilohium, the last alone including fifty different 

 kinds. This makes it all the more remarkable that so varied a family as the 

 Umbelliferse yields very few hybrids. Of the numerous hybrids belonging to the 

 Saxifragacese we may mention as specially noteworthy those derived from species 

 which differ exceedingly from one another in form and size. One cannot easily 

 imagine two plants in the limits of a single genus which present a greater contrast 

 to one another in respect of flowers, leaves, and general mode of growth than is 

 exhibited in the following cases: — Saxifraga ccesia and 8. rautata, S. Aizoon and 

 8. cuneifolia, S. aizoides and S. squarrosa, and yet hybrids have sprung from the 

 crossing of these species. 



