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P r o b Z e m s o f B i p o l a r P Z a n t D i s t r i b ·u t i o n 231rentiated and isolated within this syngameon. And even the very polymorphicsyngameon out of which they were differentiated may have flown as a broadstream from previous syngameons with a similar distribution, so very old that anyspeculations as to w hether they first came from the N ort h or from the Southwould be futile. Same special exaroples will be disenssed below (pp. 244 and 263).The general way of looking upon the genesis of taxonornie units roughlysketched above has brought me to a very sceptical attitude towards theproblem w hether a certain bi polar population has ever mig ra te d a crossthe tropics in one direction or the other.therefore, try to trace transtropical c o n n e c t i o n sIn the following discussion I shall,rather than to advance mig ra tio n hypotheses of any kind.of bipolar populations2. Long Distance Dispersal or More Continuous TranstropicalHighland Bridges?The first attempt to explain bipolar plant distribution by transtropical migTatianwas probably made by HOOKER (1844-1847). Painting out the occurrenceof Trisetum (sub) spicatum not only in the extreme South bu t also on the tropicalmountains of Peru, Colombia and Mexico, HooK'ER (1847, p. 378) found it >>evidentthat this plant, like n1any common to the opposite temperate zones, has availeditself of the direct communication afforded by the Andes of the An1erican contirrentfor migrating from the N orthern to the Southern Hemisphere>>. For bi polarspecies of this type, with intermediate occurrences on tropical mountains, he hadstated in the first part of the work (1844, p. 117) that >>the supposition of adouble creation of the same species, or of one of them being a variety of same otherreally distinct plant, which plant wholly reserobles another fron1 other countries,would be confessedly a gratuitous assumptiom>. But he immediately continued:>>Where however no intermediate stations can be detected, these suppositions becomemore plausible; the only alternatives to such conclusions being, 1st, the passibilityof the species being destroyed in the intervening positions which it mayformerly have inhabited; 2nd, the great improbability that the seed has beencarried at once from one polar region to the other ... >>Same years later HooKER (1853, p. xxv) returned to the same problem:>>With regard to the South American Alps, there is direct land communicationalong the Andes from Arctic to Antarctic regions; by which not only may thestrictly Arctic genera and species have migrated to Cape Horn, but by which manyAntarctic ones may have advanced northward to the equator.>> To this statementhe added the following foot-note: >>Why these Alltarctic forms have not extendedinta North America, as the Arctic ones have inta South America, is a curious problem,and the only hypothesis that suggests itself is derived from the fact thatthough the Panama Andes are not now sufficiently lofty for the transit of either,