The Evolution of the Kidney 43 



verted oceanic currents, altered the dust and water vapor in 

 the atmosphere, raised barriers to moisture-laden winds and 

 otherwise interfered with the basal forces that control the 

 weather, these revolutions have been accompanied by marked 

 and protracted changes in climate over the entire surface of 

 the earth. In general, periods of mountain building have been 

 accompanied by marked refrigeration so that in some in- 

 stances glaciers have descended to sea-level in equatorial lati- 

 tudes; while in the quiescent intervals, after erosion had 

 levelled the recently formed mountains to mere hills, warm 

 shallow seas have transgressed widely over the low-lying lands, 

 and even Arctica and Antarctica have enjoyed a climate that 

 was warm and humid. "^ 



According to modern experimental biology, the vis a 

 tergo of evolution is the production of new varieties in con- 

 sequence of random mutations in the chromosomes; such of 

 these varieties as are unfitted to survive are pruned away by 

 natural selection, leaving the better fitted mutants to get 

 along as best they can. Mutation is fundamental to evolu- 

 tion, but mutation itself would be of little avail to modify 

 organic pattern did not the vis a f route of natural selection 

 foster the survival of exotic individuals, of the new muta- 

 tions, by offering them a special environment in which their 

 unique characters are advantageous, by preserving them from 

 genetic extinction through backbreeding with the unmutated 

 forms, and probably in other ways. We may believe that in 

 the shaping of the final evolutionary product as we find it, 

 mutation and environment have played balanced and equal 

 roles. Though we cannot assign to either mutation or selec- 

 tion any teleological direction, they tend within certain limits 

 to have one result: after a few million years, when many 

 millions of mutations have occurred and most of them have 



