140 THE AMEBIC AN NATUEALIST [Vol.L 



in the form of a high degree of variability, accompanying 

 a considerable manifestation of sterility in the gametic 

 cells, particularly the pollen. In the numerous species of 

 Rosa or Oenothera, we find in regard to both variability 

 and the phenomenon of sterility, a marked contrast to the 

 also numerous species of the very old genus Pinus. In 

 Pinus there is practically no imperfection in the develop- 



ment of the microspores, even in exotic species, and the 

 species are very clearly marked and constant. 



If associated variability and gametic sterility are reli- 

 able indications of hybridization, then it becomes clear 

 that the Angiosperms, unlike the Gymnosperms and the 

 mass of the vascular Cryptogams, are often character- 

 ized by heterozygosis. It has been recently suggested 

 that pollen imperfection is not so much an evidence of 

 hybridization as of mutability. This criticism appears to 

 fail for various reasons. First, for nearly a hundred 

 years practically all students of hybridization in plants 

 have noted pollen sterility and imperfect development of 

 the seed as peculiar characteristics of hybrids. Secondly, 



