574 



THE GENESIS OF NEW FORMS AS A RESULT OF CROSSING. 



a Buckthorn-shrub, named Rhamnus hybrida, which sprang from a cross between 

 Rhamnus alpina and Rhamnus Älaternus. One of the parent-species, R. alpina, 

 has deciduous foliage, i.e. leaves which are green in the summer and wither and drop 

 in the autumn ; the other, R. Älaternus, has evergreen leaves, which last through 

 the winter and remain on the branches for two years. The hybrid, R. hybrida, 

 possesses leaves which do not fall off in the autumn, nor yet last fresh and green for 

 two years, but which maintain their verdure through one winter and fall in the 

 spring when new shoots are sprouting from the buds. The behaviour of hybrids 

 as regards their season of flowering is also very remarkable. From 1863 to 1874 I 

 kept notes concerning the flowering of some fifty different kinds of Willow, growing 

 in the Botanic Garden at Innsbruck, and each year made an entry of the day on 

 w^iich the first flower opened in each plant, whether a pure species or a hybrid. 



Earliest Date of Flowering of a number of Willows growing in the 

 Botanic Garden at Innsbruck. 



(The date given is the average for 12 years.) 



The name in the first column is that of a hybrid in each case, and the names on the same line in the second and third 



columns are those of its parent-stocks. 



The above table, which gives the means of the dates recorded in 12 years of the 

 first opening of the male flowers in 15 species and 17 hybrids produced from them 

 by a variety of crosses, shows that the hybrids invariably flower on days between 

 those on which the parent-species enter upon that stage of development. It will be 

 observed that the two alpine Willows, Salix retusa and Salix Jacquiniana, flowered 

 on an average in the 12 years on the same day, and that their hybrid Salix 

 retusoides kept also to that date. 



We have hitherto dealt with those of the marks, attributes, and vital phenomena 

 manifested by hybrids which are derived partly from the one parent-species and 

 partly from the other, and we must now pass to the consideration of such character- 

 istics as cannot be attributed to inheritance from those species. There is, in the 

 first place, the fact that the majority of the hybrids produced from crosses develop 



