Experimental Pedigree-Cultures 573 



that this theory of natural selection leaves the 

 question as to how the changes themselves are 

 brought about, quite undecided. There are two 

 possibilities, and both have been propounded by 

 Darwin. One is the accumulation of the slight 

 deviations of fluctuating variability, the other 

 consists of successive sports or leaps taking 

 place in the same direction. 



In further lectures a critical comparison of 

 the two views will be given. To-day I have only 

 to show that the mutations of the evening-prim- 

 roses, though sudden, comply with the demands 

 made by Darwin as to the form of variability 

 which is to be accepted as the cause of evolution 

 and as the origin of species. 



Some of my new types are stouter and others 

 weaker than their parents, as shown by gigas 

 and albida. Some have broader leaves and 

 some narrower, lata and ohlonga. Some have 

 larger flowers {gigas) or deeper yellow ones 

 (rubrinervis) , or smaller blossoms (scintillans) , 

 or of a paler hue (albida). In some the cap- 

 sules are longer (ruhrinervis), or thicker 

 (gigas), or more rounded (lata), or small (ob- 

 long a), and nearly destitute of seeds (brevi- 

 stylis). The unevenness of the surface of the 

 leaves may increase as in lata^ or decrease as in 

 laevifolia. The tendency to become annual pre- 

 vails in rubrinervis, but gigas tends to become 



