SVENSON : TAXONOMY IN RELATION TO GEOGRAPHY 45 



at least not those enumerated above as existing species. These and their fel- 

 lows, or such as survived, must have been pushed on to lower latitudes as the 

 cold advanced . . . , portions of which, retreating up the mountains as the cli- 

 mate ameliorated and the ice receded, still scantily survive upon our highest 

 Alleghenies, and more abundantly upon the colder summits of the mountains 

 of New York and New England." 



"... perhaps the most interesting and most unexpected discovery of the 

 expedition is that of two strictly Eastern North American species of this order 

 [Berberidaceae], — each the sole representative of the genus, — viz. Caulophyl- 

 liiin thalictroides, and Diphylleia cymosa, of Michaux . . . are we to regard 

 them as the descendants of a common stock ... or are we to suppose them in- 

 dependently originated in two such widely distant regions?" (p. 380). 



"Smilacina {Majantheuium) bifolia extends around the world, but under 

 three pretty well marked geographical varieties : — the European, which extends 

 to eastern Siberia; the var. Kamtschatica, which replaces the former on the 

 Pacific Siberian coast, in Japan, and in North America west of the Rocky 

 Mountains ; and the var. Canadensis, throughout all the northern part of this 

 country east of the Mississippi and the Rocky Mountains" (p. 414). 



These quotations, it will be seen, are important from three points of view in 

 our modern taxonomy : 1 ) affirmation of the idea of evolution by the natural 

 selection of variations, 2) the negation of the bicentric origin of species, 3) 

 recognition of a holarctic Cretaceous flora of common origin, and its disruption 

 by the Glacial period. 



And we arrive here at one of our first taxonomic difficulties. Shall these 

 geographic variants, which Gray showed to be of common origin, be classified 

 as a single species or shall they be segregated as separate species ? This impor- 

 tant question we cannot decide. As Weatherby' has noted : "so long as we have 

 to rely on judgment at all, the accuracy and soundness of any taxonomic cate- 

 gory, definition or no definition, will be in direct proportion to the accuracy 

 and soundness of judgment of the individuals who apply it." The pendulum 

 swings this way and that over periods of time. For example, the yellow lady's 

 slipper (Cypripedium puhescens) of eastern United States has long passed as 

 distinct, but only recently CorrelP has with some justification treated it as a 

 variety of the Eurasian Cypripedium Calceolus. The common brake of eastern 

 North America, long held as a separate species under the name Pteridium 

 latiusculum, has recently been returned by Tryon to its very old status 'as a 

 subdivision of the wide-spread Pteridium aquilinum. And in Rhodora for this 

 very month of June we find the common water-plantain, which through later 

 years we have been patiently calling Alisma snibcordata, blooming forth after 



- Rhodora 44 : 160. 1942. 



3 Harvard Bot. Museum Leaflet 7: 1-18. 1938. 



