576 THE GENESIS OF NEW SPECIES. 



Hybrids also exhibit the phenomenon known as the "doubling" of flowers, 

 which depends upon the transformation of stamens into petals, independently of 

 the action of tiny gall-mites, which are the frequent cause of doubling in other 

 plants (cf. p. 548). Several hybrid Roses, Pinks, and Camellias are only known 

 with double flowers. 



It is diflicult to explain the fact, repeatedly confirmed by observation, of the 

 appearance in hybrids of characters which are not present in either parent-species, 

 or rather which cannot be traced to inheritance from either of those species. Thus 

 it sometimes happens that individual plants of a hybrid develop sinuate foliage- 

 leaves with wavy outline, though in both of the parent-species the leaves are either 

 entire or only slightly toothed. The hybrid Salvia sylvestris occasionally exhibits 

 deeply sinuate radical leaves, whilst Salvia nemorosa and Salvia pratensis, the two 

 species to which it owes its origin, never do so. Another instance of the same kind 

 is that of a Stock, the hybrid of Matthiola incana and Matthiola Maderensis. 

 Neither the one nor the other parent-species has sinuate leaves, yet here and there 

 plants of the hybrid display foliage with the margins so deeply cut as to remind one 

 at first sight of Matthiola sinuata. Again, in Primula pubeseens the leaves are 

 sometimes more deeply sinuate than in either Primula Auricula or Primula 

 hirsuta. In hybrids of the Foxglove genus {Digitalis), flowers not infrequently make 

 their appearance wherein the corolla is produced underneath into a spur as in the 

 Toad-flax {Linaria). One hybrid produced by crossing two species of Water Lily, 

 Nympltaia Lotus and Nymphwa dentata, displayed dark violet lines on its sepals 

 which are not to be seen in either parent-species. Reference must also be made to 

 the comparative frequency with which hybrids bearing white flowers spring from 

 species with blue, violet, red, or yellow blossoms whose non-hybrid offspring only 

 produce colourless flowers on very rare occasions. Lastly, we may mention the fact 

 that as from species so also from hybrids varieties may be formed; but they 

 have no permanence amongst the descendants of a race, passing into other varieties- 

 whenever it undergoes the restrictive influence of a change in external conditions. 



3. THE OKIGIN OF SPECIES. 



Genesis of New Species. — Derivation of Existing Species. — The Sub-divisions of the Vegetable 



Kingdom. 



THE GENESIS OF NEW SPECIES. 



It is now more than forty years since I discovered, on an island on the Danube 

 not far from the little town of Dürenstein, a Willow which had till then remained 

 unknown to Botanists. The plant in question was growing on the island in com- 

 pany with a number of other Willow-trees and Willow-shrubs belonging to the 



