62 



ORGANIC EVOLUTION 



reproduction is concerned. The faunas and floras of east- 

 ern Asia and of our west coast give us possibly the best 

 example of the segregating effect of climate. At one time 

 the climate of Siberia and that of Alaska was semi-tropical, 

 being considerably more mild than the present climate of 

 Baltimore. Of this we have abundant evidence in the 

 fossil remains of semi-tropical plants and animals over 



FIG. 9. Locusts taken on the Galapagos Islands, Pacific Ocean. All descended from a 

 common ancestor, but now scattered over the various islands and differing- in size and markings. 



o o 



a. Silintoierca melanora (Charles Island), b. S. intermedia borealis (Abingdon and Bindloe 

 Islands), c. S. intermedia (Duncan Island), d. .S'. literosa (Chatham Island), e. S. melanora 

 lineata (Albemarle Island), f. .S'. melanora inimaciilata (Indefatigable Island.) The species 

 intermedia is probably a hybrid between the other two species. From Jordan and Kellogg's 

 Animal Life, by the courtesy of the authors and of D. Appleton & Co. 



this whole area. During the continuance of the warm 



O 



climate many species crossed from Asia to America and 

 vice versa across Behring straits. As the cold increased, 

 culminating in the extreme cold of the glacial period, there 

 was formed a most effective barrier to further migration 

 from one continent to the other, resulting in the complete 

 segregation into two groups of each species which had 

 representatives in both regions. We now find, as we 



