Nov., 1923] 
DAVIS-POLLEN- AND SEED-STERILITY 
467 
exceptional attention, are marked exceptions among animals and plants. 
Intensive studies on other forms are more than likely greatly to extend our 
recognition of the presence in nature of impure lines and impure species. 
Hybridization in itself probably invites the development of lethals in 
proportion as the mixing of diverse germ plasms disturbs delicate and 
vital adjustments and creates confusion in orderly processes of development. 
The subject of lethals and impure species has come to have particular 
interest for the student of certain groups of plants which are conspicuous 
components of floras. The systematics of Oenothera has reached a stage 
so complex that much material can not be identified in the field and species 
may be determined only when their behavior is studied in the experimental 
garden. Systematic studies on violets and brambles have employed 
similar methods of genetical analysis, and many other groups will require 
the same sort of treatment. Then there are those large assemblages 
characterized by high degrees of self-sterility, conditions probably not un¬ 
common in the Compositae. Here the progeny is always or usually cross¬ 
bred as in all unisexual animals and plants. Again, even when self-fertiliza¬ 
tion is possible, it has been found in some material that inbred lines are not 
so vigorous as the outbred and thus conditions favor hybridity. 
We open a paper with a discussion of pollen- and seed-sterility in hybrids. 
We are led at the end to touch upon some of the most complex problems of 
genetics and taxonomy. 
University of Michigan 
