346 THE CHANGING GENERATIONS 



unpredictable as are those caused by other mutations, and indeed many 

 of the induced mutations are the same as those that occur spontaneously 

 (though at a very much slower rate) in laboratory and wild populations. 

 By far the greater part of these and of all mutations are detrimental or 

 lethal. 



On the assumption that radiation simply accelerates the mutation 

 rate, it now becomes possible to measure rates of mutation. It is found 

 that many known genes and certain specific arrangements of chromosome 

 parts do have a characteristic even though very low mutation rate. As 

 might be expected, different genes show widely different rates. A rate of 1 

 mutation in 100,000 genes is comparatively very high; 1 per million or 1 

 per several million would be more nearly an average rate, and some genes 

 evidently have a much lower mutation rate than even this. 



When we calculate the huge numbers of genes per individual — esti- 

 mated to be between 5,000 and 12,000 in Drosophila, and from 15,000 

 to 25,000 in man — and multiply these numbers by the estimated size of 

 population and number of generations in even a short stretch of geological 

 time, it may be assumed that the supply of inherited variations is amply 

 sufficient as material for evolution. In this connection Ave should take into 

 consideration the possibilities of recombination. As different alleles 

 accumulate in the total gene complement of a stock — or to put it another 

 way, as the amount of heterozygousness of an interbreeding population 

 increases — the new combinations possible for the genotype of any given 

 individual increase exponentially with each new mutation. Even though 

 the greatest number of new mutations are detrimental and destined to be 

 eliminated sooner or later, there should still remain sufficient viable and 

 neutral or potentially adaptive new genes and new gene combinations to 

 provide an ample supply of variability upon which selection may act. 



Inheritance and the Mechanisms of Gene Assortment 



The entire field of Mendelian phenomena and of the cytological mecha- 

 nisms that account for them was developed long after Darwin's time. 

 We now know that inheritance is particulate — analyzable into the effects 

 of individual genes that (barring rare mutations) are transmitted intact 

 and unchanged from generation to generation. Ample evidence has 

 accumulated to establish the fundamental difference between germ and 

 soma and the continuity of the germ cell line. The phenomena of segrega- 

 tion and recombination and of linkage and crossing over have been 

 verified both by breeding and by cytological observation and experiment. 



Inheritance still remains as Darwin conceived it — the transmission 

 of qualities from parent to offspring, but it is now revealed as having 

 an even more effective and far-reaching role in evolutionary processes 



