No. 591] HYBRIDISM IN ANGIOSPERMS 



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multiplication of generations as well as of individuals, 

 rendered possible by the appearance of the herbaceous 

 type of small size and short reproductive cycle, will con- 

 tribute to the acceleration of evolutionary processes. 



A noteworthy feature, which distinguishes the huge 

 aggregation of Angiosperms now inhabiting the surface 

 of the globe (in the neighborhood of one hundred and 

 forty thousand species) from the saved remnant of the 

 Gymnosperms, is their inherent variability. This high 

 degree of variability has naturally made the Angiosperms 

 a very difficult group from the systematic standpoint and 

 has likewise put them in the foreground in connection with 

 discussions as to the origin of species. Two of the oldest 

 tribes of the coniferous Gymnosperms are the pines and 

 and the araucarians. I have had the good fortune to be 

 able to make a careful comparison of structure extending 

 to all important details, between living representatives 

 of these tribes and their predecessors in the Cretaceous 

 of the eastern United States. It is quite clear from these 

 studies that the genus Pinus and the genus Araucaria in 

 the remote times of the Age of Chalk, differed only in the 

 smallest particulars from their living descendants. The 

 conclusion inevitably follows that the course of evolution 

 here has been very slow. The actual situation corre- 

 sponds accurately with the data derived from the past. 

 A white pine, compared with an evening primrose or a 

 rose, is relatively constant and invariable. 



The remarkable variability of the Angiosperms, as fre- 

 quently expressed in terms of the difficulty of systematic 

 identification, brings us naturally to the much debated 

 question of the origin of variability. Darwin, as is well 

 known, simply accepted this phenomenon as a fact and 

 did not, after the first, at any rate, attempt to explain the 

 condition in terms of other phenomena. It is interesting, 

 however, to note that in the beginning he was disposed to 

 accept hybridization as the cause of the variability of 

 species and apparently abandoned this belief only because 

 he could find no evidence for its occurrence on a sum- 



