Dr. CrolPs Theory of Glacial and Warm Periods. 99 



evidence that is accumulating will force geologists to the 

 conclusion that the ice of the glacial period was not only 

 more extensive than has been generally supposed, but that 

 it existed at the same time on the northern and southern 

 hemispheres, leaving, at least on the American Continent, 

 only the lower lands of the tropics free from the icy 

 covering" {id., 264-5). Mr. Belt again says : " The glacial 

 periods probably existed in both hemispheres at the same 

 time. First, because we can trace the evidence of the 

 existence of ice along the high lands of America into the 

 northern tropics until it nearly inosculates with that coming 

 down from the south, and there is no difference in the 

 character or appearance of the moraines left on both sides 

 of the equator. Second, because, except on the supposition 

 that the ice extended at least along some meridians, both 

 from the south and the north, nearly to the equator at the 

 same time, we cannot explain the distribution of those 

 annuals and plants that are found in the temperate zones 

 of both hemispheres, separated by the whole width of the 

 tropics, over which they cannot now pass. For example, 

 there are more than forty flowering plants of North America 

 and Europe, which are also found in Tierra del Fuego. 

 Darwin's theory, that these plants were driven to the high 

 lands of the tropics during the glacial period, and followed 

 the retreating ice in its retrocession, must fall to the ground 

 if the ice did not exist in both hemispheres at the same 

 time." {Nature, X., 25 and 26.) 



Nor is this alternating theory supported by an appeal to 

 what we know of other bodies than the earth, which have 

 an orbit of marked eccentricity, and are therefore in a 

 condition like the earth must have been during the sup- 

 posed glacial period. Fortunately, as Professor Bonney 

 has suggested, it is easy to make such an appeal. The 

 planet Mars, so far as we can gather, is constituted 

 like the earth. We can trace out its land area and its seas, 



