BETWEEN RACES 151 



leaves so finely divided or dissected into minute seg- 

 ments as to resemble a camel's-hair pencil when re- 

 moved from the water. Sooner or later the growing 

 terminal bud reaches the surface, and rises above it 

 into the air. As soon as this happens, the rudimentary 

 leaves just beginning to swell within the bud entirely 

 change their course of development. They grow now 

 into flat-lobed blades, which float upon the surface of 

 the water. The change of environment from water to 

 air has worked such an alteration in form that no one 

 who was not in the secret would suppose that these 

 two kinds of leaves could possibly have been borne 

 upon the same plant. 



De Vries would say that the tendency to produce 

 the floating kind of leaf was latent in the submerged 

 plant. In other words, the appearance of any given 

 plant, or that of any given part of it, depends partly 

 upon its hereditary qualities and partly to the external 

 circumstances to which it is submitted. Many other 

 examples of similar changes could be alluded to, and 

 one recently described is of rather special interest to 

 students of genetics. This relates to a variety of 

 Primula sinensis, which, if kept at a temperature of 

 30 C., in a moist greenhouse, produces red flowers 

 only ; whilst under similar conditions, but at a tem- 

 perature of 20 C., bears only pure white blossoms. 

 Another variety of the same species exists which only 

 produces white blooms when exposed to any condi- 

 tions under which it will flower at all. In describing 

 the hereditary difference between these two varieties, 

 we cannot say that it consists in the former having red 



