Other Kinds of Hybridizing 165 



petal. "The flush is transmitted independently of the length 

 of the style or the size of the pollen grains, for it may be trans- 

 ferred to the true short-styled or ' thrum ' type. But when the 

 flush is developed in plants which by gametic composition would 

 be long-styled, the style does not pass through the anthers and 

 the equal-styled condition is produced. Why the development 

 of the yellow flush in these flowers should entail the reduction of 

 the style, we cannot in any way suggest." 



The discovery, that Primula follows the same law of inheri- 

 tance as do other discontinuous variations, some of which are 

 known to have appeared suddenly, furnishes an argument in 

 favor of the view that the dimorphism in Primula owes its origin 

 to discontinuous variation. Compared with the involved argu- 

 ment by which Darwin attempts to show how natural selection 

 has brought about the result, the mutation theory offers a much 

 simpler and in my opinion a much more plausible interpretation. 



Reversal of Symmetry 



In some species of snails the spiral of the shell turns to the 

 right, in other species to the left. Occasionally in a right- 

 handed species an individual that is left-handed is found, and 

 there can be little doubt that such a form may suddenly arise. 

 It is a discontinuous variation, but whether it is a mutation is 

 not so clear, since the result may be due to an accidental shift- 

 ing of the blastomeres on each other at the time when the 

 unsymmetrical mesoblast cell is laid down. Nevertheless, the 

 fact that entire species are characterized by the right- or the left- 

 handed condition indicates that the factor that produces the one 

 or the other result may at times be impressed on or arise in the 

 egg, and be inherited. There are also species in which some 

 individuals are right-handed, others left-handed. Here both 

 possibilities seem to exist in the egg; but whether this can be 

 referred to alternating dominance and recession, or to purely 

 local conditions that arise during segmentation, is unknown. 



Somewhat similar conditions occur in the two kinds of chelae 

 of crabs, prawns, and other decapods ; and perhaps the right- 



