134 



ZOOLOGY 



fatal to existence. The individuals homozygous for 

 them never develop at all. No selection could be more 

 rigorous than this, yet these factors have not been 

 eliminated from the stream of inheritance. They sur- 

 vive in the heterozygotes. 



The con- 8. It also appears that although individuals are 



determiners different, the determiners giving rise to them go on from 

 age to age unaltered. That they never alter is of course 

 an absurd proposition ; but they are at any rate 

 extraordinarily constant. "Original variations" modi- 

 fying the very substance of the reproductive cells are 

 decidedly rare, instead of occurring all the time, as was 

 once supposed. The constancy of these elements is 

 shown not merely by the experience of breeders, but 

 also and more convincingly by the record of the rocks. 

 Fossil remains millions of years old show us that certain 

 forms of life, though continually subjected to "natural 

 selection," have remained substantially unchanged. 

 Even their habits have scarcely altered. Others, of 

 course, have been greatly modified, but change seems 

 not to have been obligatory as a consequence of the 

 selective process. 



9. All these considerations appear to weaken the 

 theory of natural selection as an effective cause of 

 evolution, but in reality they simply modify our idea 

 of its manner of operation. Unquestionably some 

 types are more "plastic" than others, and are more 

 quickly molded by selective agencies. Those organisms 

 whose life is very simple, who require "but little here 

 below," do not quickly change. There is no direction 

 in which they can readily improve. Bacteria, for ex- 

 ample, have apparently existed for fifty million years, 

 without important structural changes. Many species 

 have developed, adapted to particular modes of life, 



Conserva- 

 tive and 

 plastic 

 types 



