ORGANIC EVOLUTION DAVENPORT 425 



favorable mutations, it still roinaiiis to consider Avhat light modern 

 ^enetical studies throw upon the details of this process. 



"We have seen that roprodnotion is unlimited and that mutation 

 is constantly occurring; not, to bo sure, in infinite amount in all 

 directions equally but abundantly in certain penes; more rarely in 

 others. AVe have now to consider in turn what induces mutations; 

 what fjives direction to nnilations; how are the a<laptations in natui-e 

 broujrht about? 



First, the causes of mutation are clearly determined, in part, by 

 (he unstable nature of (he «zi>ne itself. The genes are very complex 

 molecules, or rather groups of molecules. Now, many complex 

 molecules are known in chemistry that are so labile (hat they break 

 down almost spontaneously. How many compounds have to be 

 kept in the dark, at a low temperature, undisturbed in order that 

 they may "keep" at all. In time they "spoil," even under the best 

 of conditions. The rate of cliange may be accelerated by heat, light, 

 and irradiation. The genes are not exceptions to the rule of change 

 in labile substances. "While the nature of the process of mutation 

 in tlie genes has shown itself little influenced by external agents the 

 velocity of spontaneous change is readily altered. Thus MuUer, 

 Hanson, and others find the speed of mutation accelerated by in- 

 crease of temperature, and by X rays. No new mutations have been 

 brought forth by the X rays, but mutation occurs more rapidly. The 

 X rays do not direct evolution, or even mutation, qualitatively, but 

 afford opportunity for more rapid evolution by providing, in larger 

 amount, one of the factors — mutation — necessary to evolution. 



The fact that X rays may accelerate the process of gene mutation 

 has led some to the conclusion that all mutation is due to radiations. 

 No doubt conditions compatible with the life of the soma may be 

 found that will not permit of mutation, but it does not seem ]irob- 

 able that this will ever be the case. Just the conditions essential 

 to life probably provide the conditions for gene mutation. 



Second, what gives direction to mutations? It is obvious that the 

 organic world is far from being the infinitely diverse collection of 

 haphazard and meaningless variants that we might expect were 

 mutation entirely uncontrolled. It seems probable that, under ordi- 

 nary conditions, genes break down or disintegrate in orderly fashion 

 dependent on the nature of the gene. An illustration, which may be 

 more than a mere illustration, is afforded in the rare earths where 

 uranium breaks down into radium and radium into lead. Each 

 stage, in turn, is determined by the immediately preceding stage. 



So, in organisms, the mutations are usually of a recessive nature, 

 which seems generally to imply that they are produced bj^ a loss of 

 something from the parental gene. In the different species of one 



