lo POPULAR SCIENCE MONTHLY. 



THE WEST INDIAN BRIDGE BETWEEN NORTH AND 

 SOUTH AMERICA. 



By Dr. J. W. SPENCER. 



MUTABILITY OF OcEANS AND CONTINENTS. — A glaiice at a map 

 of the American continent, inclosing the West Indian seas 

 within its mass, suggests that these basins are sunken plains, sub- 

 merged to only a moderate extent, but the soundings show depths 

 reaching to more than three miles. " It is not too much to say 

 that every spot which is now dry land has been sea at some former 

 period, and every part of space now covered by the deepest oceans 

 has been land." * This enunciation still held place among the 

 latest writings of the great geological teacher — Sir Charles Lyell. 

 As the earlier geologists had not the means of measuring the 

 amount of terrestrial movements, the doctrine of mutability of 

 continents and seas, as taught by Lyell, was doubted by many 

 who later substituted the hypothesis of their permanency from the 

 most remote times, although subjected to ceaseless changes of form. 

 The hypothesis of permanency of continents and seas was largely 

 based upon the littoral character of sedimentary formations, although 

 the evidence of the abysmal or oceanic origin of the widespread 

 chalk deposits could not be easily disposed of. Again, the develop- 

 ment and distribution of animal and plant life have been skillfully 

 used as evidence against certain great changes in insular and con- 

 tinental connections, beyond limited proportions. The amount of 

 the concession has varied greatly among the different advocates, so 

 that even under the general hypothesis of permanency, the con- 

 figuration of the West Indian region has undergone great changes, 

 yet not sufficient to bridge over the seas between the two Americas. 



The biological evidence alone, in favor of the permanency of 

 continents and oceans, sometimes suggesting the most startling evi- 

 dence to the contrary, is insufficient to base theories upon, unless 

 supported by physical indorsement. Indeed, some of the most inter- 

 esting questions concerning the distribution of biological forms in 

 the West Indian seas can only be explained by the recent discoveries 

 of the physical changes of the region. Some of the smaller oscilla- 

 tions of land and sea have often been measured, but the determina- 

 tion of many of those of stupendous proportions, while occasionally 

 hinted at, was not accomplished until the standard value of the great 

 yardstick was found in the West Indian seas. The discovery of the 

 Antillean bridge between North and South America, which is the 



* The last edition of Lyell's Principles of Geology. 



