300 BROOKLYN BOTANIC GARDEN MEMOIRS 



are not able to survive successfully they are found to the exclusion of 

 one or both of these. This last conclusion upsets the conventional 

 assumption that hybrids can only exist where their originating species 

 occur side by side. It is clear from the general results of the highly 

 important systematic and geographical investigations of Kerner that 

 new species m,ay appear as the result of spontaneous hybridization. 



The more recent evidence supplied by the investigations of Brainerd 

 upon the violets and certain Rosaceae point equally positively in the 

 same direction. This author has made it clear that a number of 

 recognized species of Viola and Ruhus are in reality hybrids in their 

 origin. A particularly interesting result reached by Dr. Brainerd is 

 that these hybrid species may become absolutely fixed in spite of their 

 mode of origin and respond not only to recognized systematic but also 

 to genetical criteria for species. 



It is too often assumed at the present time that the best criteria 

 of species are physiological. On this basis the capacity to breed true 

 in cultures and to produce offspring which comply with the tests of 

 genetical analysis is regarded as of paramount importance. Since 

 many known hybrids comply equally with recognized species with 

 these tests it has become clear that what a plant does in cultures can- 

 not be accepted as an infallible evidence of its antecedents. Where 

 physiological criteria fail, we turn to the more constant ones furnished 

 by morphological characters. It has been recognized for nearly a 

 century that sterility is often a marked feature of hybrids, especially 

 when they result from the crossing of somewhat incompatible species. 

 The causes of incompatibility are apparently unknown as often 

 species more different in their external characteristics and more 

 widely separated in geographical range can be crossed with greater 

 success than those nearly related on the evidence of external features 

 and geographic coincidence. For example the horse and the zebra 

 produce fertile hybrids, while the horse and the ass, when crossed, give 

 rise ordinarily to infertile mules. Similarly our common canoe, yellow 

 and black birches, which often grow side by side without hybridizing 

 all apparently cross with a considerable degree of readiness with the 

 more isolated shrubby birch of swamps, Betula puniila, according to 

 the investigations of Jack and Rosendahl in this country. 



Hybrids may present in the case of plants a number of interesting 

 morphological characteristics. For example we frequently find a 

 high degree of imperfection in their gametic cells, male and female, 

 with the emphasis of sterility nearly always on the male. This feature 

 is often so marked that it is impossible to fertilize a hybrid with its 

 own pollen, even when the ovules present a considerable degree of 

 fertility. The morphological imperfection in pollen grains can obvi- 



