400 The Inward Urge to Change 



De Vries' theory of mutations, elaborated by later ex- 

 perimental research, accounts for the origin of hereditary 

 departures from ancestral models. It does not stand to 

 reason — that is, it is not in accord with our familiar ex- 

 perience — that one " kind " of living thing should give birth 

 to another " kind." //, however, there are mutations, and 

 // they reproduce their novel characters, and // the new types 

 are capable of maintaining themselves under the prevailing 

 conditions, then the theory explains evolution, in the sense of 

 descent with modifications. Unfamiliar as the concept of 

 progression by jumps is to most of us, we may only ask, 

 What are the facts? As we have seen, the mutation theory, 

 within the limits of its more cautious advocates, is quite in 

 accord with the facts of observation and experiment. The 

 theory does not account for the origin of life nor for the dis- 

 tinctive properties of protoplasm. Especially does it fail 

 to throw any light upon the origin of adaptations. 



Reconciling Divergences 



The advantages and shortcomings of these leading evo- 

 lution theories have naturally lead to a great deal of con- 

 troversial discussion among the specialists. And occasionally 

 those of us who are not so well informed also take a hand 

 in it. There are some who feel that every serious idea which 

 seems plausible must have some " truth " in it. They ac- 

 cordingly attempt to reconcile theories that appear mutually 

 incompatible — just as we sometimes struggle to reconcile 

 our prejudices or preconceptions with the obstinate and 

 embarrassing facts. Natural selection at least can be har- 

 monized with mutation. We have seen that de Vries him- 

 self believed his contribution to consist of showing that the 

 variations upon which natural selection acts as a sieve origi- 

 nate by mutations or jumps. Whichever theory one may 

 favor, however, all have certain things in common: They all 

 seek to formulate a cause-and-effect principle which will ac- 

 count for the progressive modification of living forms in the 



