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ia iB Geographical Distribution of Indigenous Plants. 67 
Prof. Gray has made it well known that there is far more resem- 
_ blance between the plants of the Atlantic coast of the United States, 
and the Pacific coast of Asia, than between the latter and the Pacific 
coast of America, especially of California. But this can not be con- 
sidered so very remarkable when we come to note the very marked dif- 
ference in the climate between the two sides of the continent. The 
Eastern and the Asiatic climates resemble each other very much more 
than the Californian. In the first two we have four well marked sea- 
sons, characterized by abundance of moisture, while in California there 
are but two, the wet and the dry, as in tropical countries. The dry 
season is So severe as to cause the death of all plants for whose growth 
continual moisture is necessary; and we find that the majority of plants 
common to eastern North America and western Asia, but absent from 
California, are among the very ones for whose growth, moisture, and 
especially shade, isnecessary. Of the latter, many parts of California 
are utterly destitute. In reality we have, in California, a continuation 
of the climatic conditions existing in Mexico and the semi-tropical 
parts of America; and not only is the flora closely allied to that of 
Mexico, but the fauna also. It forms a separate region in Wallace’s 
“ Distribution of Animals,” and must, in any general classification, be 
separated from the rest of America, because of many peculiarities. We 
have, therefore, many good reasons for not finding there more than 76 
species out of 258 living in America and Asia. 
To sum up our observations, then, we find: 
lst. That the time necessary for the distribution of our plants has 
been sufficiently long. 
2d. That the species of plants common to Europe and America have 
had a common origin in the land about the North Pole. 
3d. That they have migrated south on account of the cold in the 
Arctic regions. 
Ath. That on account of present climatic conditions, some species 
reaching a high latitude in Europe are not found in America as far 
north by 20 deg. 
5th. That the chain of the Rocky mountains, and the Andes, fur- 
nishes, or has furnished, a highway for the dispersion of some Arctic 
forms over the southern hemisphere. And 
6th. That the similarity between the floras of Europe, of Northeast 
Asia, and Eastern America, is greater than that between Asia and 
the American Pacific coast, on account of the great difference in cli- 
matic conditions, and because of the closer connection which exists be- 
tween California and the semi-tropical region of Mexico. 
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