432 THE NATURAL HTSTORF [chap, xxxix. 



of land and sea on a large scale, and at a rate which, 

 measured by the time required for a change of species, 

 must be termed rapid. By speculating on such changes, 

 we may easily see how partial waves of immigration may 

 have entered New Guinea, and how all trace of their 

 passage may have been obliterated by the subsequent dis- 

 appearance of the intervening land. 



There is nothing that the study of geology teaches us 

 that is more certain or more impressive than the extreme 

 instability of the earth's surface. Everywhere beneath our 

 feet we find proofs that what is land has been sea, and 

 that where oceans now spread out has once been land ; 

 and that this change from sea to land, and from land to 

 sea, has taken place, not once or twice only, but again and 

 again, during countless ages of past time. Now the study 

 of the distribution of animal life upon the present surface 

 of the earth, causes us to look upon this constant inter- 

 change of land and sea — this making and unmaking of 

 continents, this elevation and disappearance of islands — as 

 a potent reality, which has always and everywhere been in 

 progress, and has been the main agent in determining the 

 manner in which living things are now grouped and scat- 

 tered over the earth's surface. And when we continually 

 come upon such little anomalies of distribution as that 

 just now described, we find the only rational explanation 

 of them, in those repeated elevations and depressions which 



