THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 597 



species without much common biocHmatic history. Thus the Laurentian 

 goldenrod association ( Sohdaginetum laurentianum ) , now a fairly har- 

 monious (although admittedly heterogeneous) community, consists of 

 North American natives and recently introduced European elements. 

 Likewise the wire-birch forest has as its dominant tree an Atlantic 

 Coastal Plain species now associated with many boreal species! 



The second hypothesis is the more or less local persistence of a 

 very ancient association which more or less recent climatic change has 

 otherwise forced to disintegrate into several parts. Such may be the 

 Thuja occidentalis association, a species-poor community probably 

 cornered out of a rich, temperate rain forest of the Tsuga-Thuja type 

 now prevaihng on the North Pacific Coast of North America. 



Fossil remains of Tertiary and Pleistocene communities provide 

 evidence of the composition and structure of earlier states of modern 

 communities. Cain (1944) thinks that constancy is a better indication 

 of dominance than local abundance. He makes many useful compari- 

 sons between fossil vegetations and their modern counterparts. Sharply 

 defined modern associations in areas marked by seasonal climates may 

 well have emerged out of relatively "undi£Ferentiated" communities of 

 modern cHmates. Lucy Braun (1935, 1938, 1941, 1950) was the first to 

 suggest the development of "association-segregates," apropos of the 

 deciduous forest of Eastern North America. In the mountains of Brazil 

 I was struck with the wide applicability of this hypothesis (1947). Later 

 (1952b, 1957b) I ventured to consider the origin of the deciduous 

 forest complex itself and suggested that it had emerged through associ- 

 ation-segregation from temperate rain forest. 



In fact, the remnants of temperate rain forest at the mid-latitvides, 

 if compared among themselves and with Mid- and Late-Tertiary de- 

 posits, allow of such an interpretation. The Canary Islands, Madeira, 

 and the Azores still harbor almost intact and certainly very pure and 

 cohesive stands of laurel-type forest. These are, however, lacking in 

 many elements present in the Pliocene of Europe, especially the pre- 

 Dicotyledon elements, such as Sequoia and Ginkgo. On the other hand, 

 the Mexican cloud forest harbors a number of deciduous-forest ele- 

 ments {Liquidambar, Acer, Fagus, which are not as fully deciduous as 

 in Eastern North America), together with Podocarpus, Clethra, 

 Rapanea, and other exclusively temperate rain forest elements. 



Is this a "before" or "after" phenomenon? Is this one step back 

 from the "mixed mesophytic"? Can the following equation be accepted? 



Mexican cloud forest Mixed mesophytic 



Mixed mesophytic Beech-maple, etc. 



In fact, there is good reason to suppose that many of our modern 



