Recent American Botany

Abstract

MR. M. L. FERNALD has published a very interesting review of the birches belonging to the groups Betula alba and B. nana. These trees and shrubs inhabit the northern regions of both hemispheres, and Mr. Fernald recognises in. America seven species and seven varieties, of which six species and five varieties are common to the Old World. Thus, contrary to the opinion of some recent authors, the American white birches are mostly non-endemic, though exhibiting numerous apparently distinct forms. Not only is this true, but the admitted species intergrade all along the line. “It is quite possible to trace by a series of specimens a direct connection between the dwarf Betula naha or B. glandulosa and the tall B. alba. … But since it is obviously impracticable to regard, all these forms as one species, it seems wiser to recognise the; more marked centres of variation as species which are admitted; to pass by exceptional tendencies to other forms ordinarily distinguished lby marked characteristics” (p. 189). This, of, course, brings up the question of the. definition of species. The present writer has been accustomed to use the accompanying! diagram in teaching biology. The line a a represents a species which is slightly dimorphic, as is indicated by the two prominences. The line b b represents strongly dimorphic species, connected (at b) by very few intermediates. The line c represents a case in which the intermediates have died out, and there is a complete break (at c) resulting in the formation of two Species. It is now to be pointed out that this break must be spacial or geographical, and not merely morphological, otherwise the two sexes of the same species would often have to be regarded as distinct species. Such a break need not be geographical in the ordinary sense, but when the twb species inhabit what is nominally the same locality, they are found to be differently related to their environment, or related to different closely adjacent environments. Furthermore, they must breed true, and not ordinarily interbreed one with another.