122 



SCIENCE 



[N. S. Vol. XLIII. No. 1100 



lopes and meadow-mice vanished from 

 South America, though persisting in North 

 America. The permanent contribution of 

 South America to the northern fauna is, 

 on the contrary, quite insignificant. 



"While it is impossible to say with any 

 certainty just why the northern animals 

 should have thus predominated over the 

 southern, it is reasonable to conclude that 

 it was due to the higher stage of intelli- 

 gence and structural development which 

 the former had attained. There can be no 

 question as to this structural superiority, 

 but such superiority, as we call it, does not 

 always insure victory in the struggle for 

 existence, victory largely depending upon 

 the nature of the environment. It must 

 be remembered that by no means all of the 

 northern animals which invaded Notogsea 

 were of North American origin; many 

 were immigrants from the Old World, of 

 different geological dates of arrival in the 

 western hemisphere and correspondingly 

 different degrees of modification. The re- 

 peated jimetion of North America with 

 Atia. made the former a part of Arctogsea, 

 incomparably the greatest land area of the 

 globe, and such immensity of connected 

 lands is favorable to the higher evolution 

 of terrestrial life. The comparative isola- 

 tion of South America kept that continent 

 in a relatively backward state. 



(2) The very different degrees of like- 

 ness and unlikeness between the faunas of 

 the two continents in the successive geolog- 

 ical epochs point unmistakably to extensive 

 geographical changes. Beginning with a 

 time of radical difference, when the two 

 faunas had almost nothing in common, the 

 story goes on to tell of a period when an 

 exchange of mammals began, gradually ex- 

 tending until a very considerable number 

 of types common to both continents was 

 found, and finally reducing the number of 

 these common types to the present condi- 



tion. The obvious interpretation of these 

 facts is that the complete dissimilarity of 

 animals was due to an equally complete 

 separation of the continents and that a cer- 

 tain degree of likeness was brought about 

 when a land connection was established. 

 Well-founded as this conclusion appears 

 to be, confirmation from a quite independ- 

 ent line of evidence would be welcome and 

 such evidence is to be obtained from the 

 geology of Central America and the 

 Isthmus of Panama. 



(3) Intermigrations between the Ameri- 

 cas have always had to contend with seri- 

 ous obstacles and difficulties; otherwise, 

 the interchanges of land animals would 

 have been much more extensive than they 

 ever became. Then, too, it would seem 

 that there have been times in the piist 

 when migration was less difficult than it is 

 at present, for now there is little or no 

 indication of such movements. As was 

 previously shown, the principal barrier to 

 the spread of mammals over connected or 

 continuous lands is climate, and it follows 

 that the times of migration were those of 

 more favorable climatic conditions. In the 

 early Miocene, when the migrations prob- 

 ably began, the climate was much milder 

 than at present, with less difference be- 

 tween the tropical and temperate regions. 

 Through nearly the whole of the Tertiary 

 period these conditions persisted, though 

 with a slowly progressive refrigeration, 

 and even in the Interglacial of the Pleisto- 

 cene, the amelioration of climate was such 

 that migration was rendered more prac- 

 ticable than it is under existing circum- 

 stances. 



The geological structure and history of 

 the lands around the Caribbean Sea are 

 not nearly so well known as would be de- 

 sirable, but certain very significant facts 

 have already been ascertained. Especially 

 is this true of the Isthmian Canal Zone, 



