132 DARWINISM AND THE PROBLEMS OF LIFE 



hundreds of thousands of years, been transformed from 

 an arctic to a tropical organism, from one with a thick 

 fur to one with a thin coat, it is clear that a fresh 

 lowering of the temperature will be " sudden " even 

 if it takes several thousand years ; this period would 

 not suffice to bring back the thickness of the fur which 

 it took such a long time to abolish. Just as we speak 

 in geology of "recent" times, although they may be 

 hundreds of thousands of years away, so in the 

 geological sense the word "sudden" may involve 

 enormous periods. It means merely so rapid that 

 even favourable variations have not been able to meet 

 the requirements of the change. 



One more question. Can natural selection do every- 

 thing, if it has time and material enough ? Is it 

 omnipotent in this respect? Can it transform any 

 aquatic animal into a terrestrial one, and provide any 

 land-animal with wings if it becomes necessary ? This 

 is answered in the negative by most scientists. Many 

 of them grant some species the capacity of developing 

 and deny it to others. They say there are four different 

 types of animals : persistent, elastic, rigid and plastic. 

 The persistent types retain their form for immense 

 periods ; there are still in our seas organisms that can 

 be found in almost the same form in the oldest strata. 

 Elastic types tend to revert always to the earlier form : 

 rigid types have only a very slight power of adaptation, 

 and generally perish when their environment changes : 

 plastic types continually assume new forms as the 

 conditions change, and thus conform themselves into 

 new species. 



