54 Merriam Geographic Distribution of Life. 



spectable proportions : the mahogany, the mastic, the royal 

 palm, the mangrove, the sea-grape, the Jamaica dogwood, the 

 manchineel, and other species here become considerable and 

 important trees." (Forests of North America, 10th Census, 1884, 



p. 6.) 



From what has been said it appears not only that Wallace's 

 statement that " between frigid Canada and subtropical Florida 

 there are less marked differences in the animal productions than 

 between Florida and Cuba " is wholly incorrect, but that there 

 exists in Florida a well marked subtropical fauna and flora con- 

 sisting in the main (except in the case of terrestrial mammals 

 and reptiles which could not reach it) of genera, and largely of 

 species, identical with those of Cuba. This being the case, is it 

 not fair to turn the tables and ask Wallace what constitutes the 

 barrier that so effectually holds back hundreds of genera and a 

 multitude of species of Antillean or Tropical American plants, 

 insects, land mollusks, and birds now inhabiting subtropical 

 Florida ? The deep arm of ocean between Florida and Cuba or 

 the Bahamas has proved ineffectual in checking their dispersion. 

 What is the more potent barrier that prevents their northward 

 spread along the continuous land of the peninsula ? The answer 

 is summed up in the single word climate. The temperature of 

 the period of growth and reproduction in the northern parts of 

 Cuba and the Bahamas is the same as in subtropical Florida, 

 but to the northward it falls off rapidly. 



Respecting Wallace's statement that the difference between 

 the faunas and floras of hot and cold countries " is by no means 

 constant," and does not bear " any proportion to difference of 

 temperature," it need only be said that no phenomenon of nature 

 is more constant, and that the differences observed depend di- 

 rectly upon temperature. President D. S. Jordan has said : " In 

 many groups anatomical characters are not more profound or 

 of longer standing than are the adaptations to heat and cold." 

 (Popular Science Monthly, XXXVII, Aug., 1890, p. 506.) 



That " life is distributed in circumpolar zones, which conform 

 with the climatic zones, though not always with the parallels of 

 the geographer "" is a law recognized by Humboldt, Wagner, 

 Agassiz, Dana, De Candolle, Allen, and nearly all writers on dis- 

 tribution except Wallace. This law does not imply that the 

 same species, genera, or higher groups recur under the same 



