304 BROOKLYN BOTANIC GARDEN MEMOIRS 



tion by reason of their coincidence of flowering periods, is actually the 

 result of previous specific crossing. This view of the matter is strongly 

 confirmed by the fact that the investigations of Brainerd and others 

 on that genus have revealed a large number of natural hybrids. 



For a parallel case we may now turn to the genus Ranunculus. 

 If any of the species which flower in the early summer are examined, 

 such as R. acris, R. repens, R. aquatilis, R. Cymhalaria, a considerable 

 proportion of imperfect pollen is usually present and frequently the 

 grains vary extremely in size. This situation is shown for R. acris 

 in Fig. 6, Plate V. Obviously there is a great range of size in the 

 grains and some are imperfect. This condition is most naturally 

 interpreted as a consequence of previous hybridization. Fig. 5, Plate 

 V, illustrates the condition of the pollen in R. rhomhoideus, a species 

 which flowers in the very early spring long before the other species 

 of the genus have opened their blossoms. The numerous grains 

 shown in the illustration are clearly well developed and somewhat 

 uniform in size. In the species under consideration as well as in R. 

 odoratus, perfection in development of pollen is clearly correlated with 

 isolation from possibility of contamination with other species. 



In view of the facts supplied in the above instances, which might 

 be almost indefinitely multiplied in representatives of other angio- 

 spermous families, it appears clear that the obvious interpretation 

 of pollen sterility is to be adopted, namely as an indication (where 

 it occurs under normal conditions of growth) not of mutability but 

 of previous hybridization. The large situation which is so briefly 

 illustrated by the accompanying photographs, indicates the necessity 

 of bringing morphological criteria relating to the cytology and develop- 

 ment of the gametic cells (pollen and embryo sacs) into court, as well 

 as the data derived from genetical behavior, in speculations in regard 

 to the origin of species. 



It seems clear from the evidence supplied on the systematic and 

 phytogeographical sides on the one hand and that from the standpoint 

 of morphology on the other, that the crossing of species in nature is 

 an extremely common cause of the multiplication of species. It is 

 further obvious that physiological and genetical criteria must not be 

 given greater weight than the more reliable ones supplied by actual 

 history and by morphological structure, in the all-important biological 

 question of the origin of the species. It is finally apparent that the 

 genetical status of the Oenotheras is so dubious that they cannot be 

 brought into court to furnish decisive evidence in favor of the muta- 

 tion hypothesis of De Vries. It may be added in conclusion that the 

 multiplication of species by hybridization does not by any means 

 invalidate the Darwinian hypothesis but merely supplies an additional 



