V. GRANT 51 



lating mechanisms, which prevent or greatly inhibit gene ex- 

 change with other species. 



These characteristics are clearly displayed in most groups of 

 vertebrate animals. The same facile generalization can not be 

 made for the higher plants. The circumstance that the biological 

 species concept has been almost entirely a product of work in 

 systematic zoology and preeminently in vertebrate systematics 

 is a reflection of this fact. The only botanist who contributed in 

 an important way to its development in the early period was Du 

 Rietz (1930). Botanists as a whole have accepted the biological 

 species concept much less generally than have the zoologists. 



Zoologists are inclined to believe that the difficulties of the 

 species concept in plants are confined to a few exceptional cases, 

 i.e., Fisher (1954, p. 87). To the botanist, on the other hand, it 

 frequently appears as though the clear-cut species is the excep- 

 tion and the taxonomically critical group is the rule. So charac- 

 teristic is this impression that one plant taxonomist even felt 

 obliged recently to apologize for publishing an excellent mono- 

 graph of a large genus in which there were no outstanding diffi- 

 culties of species delimitation. The true state of affairs in higher 

 plants should be expressed in quantitative terms. This large task 

 can only be commenced here. 



In the California flora 32 of the families of seed plants have 

 four or more genera and 25 or more species in all. Every one of 

 these 32 large or middle-sized families contains critical groups in 

 which the species are not clearly defined. In addition many 

 smaller families in this flora also present real problems of species 

 delimitation, i.e., the Typhaceae, Iridaceae, Juncaceae, Fagaceae, 

 Salicaceae, Nyctaginaceae, Apocynaceae, Loasaceae, Lobeliaceae. 

 The species concept thus runs into practical difficulties in all the 

 large families, all the medium-sized families, and many of the 

 small families in the California flora. 



Similarly, there is a species problem of varying proportions in 

 nearly all the large angiospermous genera of the north temperate 

 zone. To list them all, from Achillea to Zinnia, would be a tire- 

 some task. If by "good species" is understood those entities which 



