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Range of 



Southern White Cedar 



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Heavy stand of 

 Southern White Cedar 



RELATED INDIVIDUALS IN SCATTERED REGIONS 



This "southern white cedar" (Chamaecyparis thyoides), growing in Connecticut, is 

 really a kind of cypress and grows on the margins of swamps. But swamps are in- 

 frequent to the north of central New Jersey, while the plant is found on their margins 

 as far as Maine. How can we account for the distribution of this species in such 

 widely separated areas? 



The prickly-pear cactus, which is common only in desert regions, is found 

 also in a few isolated barren localities in the highlands of New York and north- 

 ern New Jersey. How is it that we find these plants so far removed from their 

 normal range .^ One suggested explanation assumes that after the retreat of 

 the last glacier, warm, arid conditions prevailed throughout most of the 

 northeastern United States, so that plants requiring more moisture died out; 

 only desert plants survived, and they spread all the way from New Mexico to 

 New York. With changing conditions other plants have replaced the cacti 

 over most of the area. Another situation which is similarly explained is the 

 presence of the southern white cedar on the margins of fresh-water swamps in 

 lower New York State; this is a species that normally ranges southward from 

 the Carolinas. 



Barriers to Migration Species living on a highland, or on a continent 

 widely separated from other continents by oceans, have little chance to visit 

 other lands. The living forms or types found in such isolated places are often 

 unique. For example, British explorers found no placental mammals in Aus- 

 tralia in the decades after it was discovered by Captain Cook. This was not a 



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