The Theory of Evolution 221 



hybrid. If all hybrids followed Mendel's law, which they do 

 not in fact, we should expect a resolution of the parent types in 

 the second generation. Since this does not occur in most of the 

 mutations of cenothera, we must infer that if the mutation is a 

 hybrid it belongs to the class of stable hybrids. This is in sub- 

 stance de Vries's interpretation. If, as de Vries assumes, the 

 mutant is the result of a mutating germ-cell meeting one of the 

 ordinary kind and producing a stable hybrid, it might appear 

 that if the hybrid were back-crossed with the parent, the off- 

 spring should be again like the hybrid, but this was not the 

 result found. These and other considerations show that there are 

 some obscurities concerning the origin of the cenothera mutants. 

 Their behavior when hybridized is also difficult to harmonize 

 with other cases of discontinuous inheritance. 



Variation and Mutation in Helix 



In connection with his experiments on snails Lang has made 

 some important observations on the relation of continuous 

 (or fluctuating) and discontinuous variation. He has found that 

 in certain colonies of Helix hortensis, in the neighborhood of 

 Zurich, only two kinds of individuals exist: yellow bandless 

 and the yellow five-banded. When inbreeding they produce 

 only these two types again, the bandless condition dominating 

 in the first generation as already explained. No intermediate 

 forms appear. This is an example of strictly discontinuous 

 inheritance. 



If the banded individuals in other colonies are closely 

 studied, it is found that the number and character of the bands 

 vary ; they are broader or narrower, they may remain separate or 

 become united, especially the fourth and fifth bands. Their 

 color may be dark or light. An examination of colonies from 

 other localities shows that all transitions exist from the most 

 highly developed five-banded condition to a bandless individual. 

 Experiments establish that the offspring of the same parent may 

 show these extremes. Here there seems to exist a fluctuating 

 variability whose extremes include the two types of discontinuous 



