590 THE GENESIS OF NEW SPECIES. 



the distribution of each of the three Water-lilies in question is determined by their 

 not being able to ripen fruits beyond that limit. Of the three, Nuphar luteum flowers 

 the latest, and therefore its fruits are also the latest to rir -^n, whence it follows 

 that it is the first to fall behind; that is to say, it reaches the northern limit of 

 distribution sooner than the others, and ceases to ripen fruit in regions where the 

 others are still able to do so. But Nuphar pumiluTYi and N. intermedium are also 

 different from one another in this respect. In Norbotten and Lapland Nuphar 

 intermedium ripens its fruits a little earlier than N. pumilum, and it is conse- 

 quently able to extend rather further north than N. pumilum. The further north 

 plants go, the shorter becomes the time allotted for the performance of their annual 

 work; and those w^hich ripen their fruits early have a great advantage over those 

 which ripen later. Concerning Nuphar intermedium., it has also been ascertained 

 that the individual plants produced in nature are more fruitful than those 

 reared in gardens from artificial crosses. In the case of plants obtained in this 

 manner in the Botanic Gardens at Königsberg each capsule contained from 15 to 

 18 fertile seeds, whilst capsules ripened in the small lakes of the Black Forest 

 contained from 38 to 63, and others taken from plants growing in Lapland con- 

 tained from 41 to 72 such seeds. From these data we may infer, in the first place, 

 that N. intermedium is most prolific in situations beyond the range of the parent- 

 species; and, secondly, that it would be wrong to suppose that because a' hybrid 

 may be comparatively infertile or actually sterile in a particular locality, such 

 infertility is a characteristic of the plant wherever it may occur. 



As may be gathered from the above account of these three examples, the advan- 

 tage which a hybrid may possess over the parent-species, whereby it is enabled 

 to subsist and multiply side by side with those species, is not always of the same 

 kind. In one case it is the more vivid coloration of the flowers, in another the fact 

 of the hybrid being better adapted to a particular state of the ground, w^hilst in 

 the third the earlier ripening of the fruits, which enables the hybrid to stand a 

 more rigorous climate, gives the requisite advantage. These do not, of course, 

 exhaust, hj a long way, the possible sources of superiority, and there are many 

 instances of hybrids which thrive better than the parent-species when the climate 

 becomes milder, moister, or drier, as the case may be. It is obvious that of all the 

 different advantages which may come into play those connected with climatic con- 

 ditions are the most important, and the genesis of hybrids is probably most fre- 

 quently due to the operation of this kind of advantage. 



Far too little significance has been attached to the fact that the greater number 

 of hybrids are not found in districts where the parent-species grow together with 

 equal luxuriance, but occur where one or other of those species is meagrely repre- 

 sented, owing to the climate not being favourable to its distribution. Again, large 

 numbers of hybrids are found in parts where the boundaries of several species 

 coincide. In Europe such regions exist in the strips of land where the advance- 

 posts of the Floras of the Baltic and Black Sea, and the Floras of the Baltic and 

 the Mediterranean, respectively, encounter one another, and particularly in the 



