PRESENT CRISIS IN EVOLUTIONARY THOUGHT 27 



and vital adjustments in proportion relative to the degree 

 of protoplasmic confusion, and this means chemical and phys- 

 ical disturbance." {Science, Feb. 3, 1923, p. 111.) 



But crossing produces, in the second filial generation (F2), 

 pure (homozygous) as well as hybrid (heterozygous) forms. 

 In some cases these pure forms are new, the phenotype being 

 different from that of either pure grandparent. Such a result 

 is produced by random assortment of the chromosomes in 

 gamete and zygote formation, and occurs when the genes for 

 two or more pairs of contrasted characters are located in dif- 

 ferent chromosome pairs. The phenomenon is formulated in 

 Mendel's Second Law, the law of independent assortment. The 

 novelty, however, of the true-breeding forms thus produced 

 is not absolute, but relative. There is no origination of new 

 hereditary factors. It is simply a recombination of the old 

 genes of different stocks, the genes themselves undergoing 

 no intrinsic alteration. The combination is new, but not the 

 elements combined. In addition to chromosomal recombina- 

 tion, we have factorial recombination by means of crossovers. 

 This, too, can produce new and true-breeding forms of a fixed 

 nature, but here, likewise, it is the combination, and not the 

 elements combined, which is new. The "new" forms thus pro- 

 duced are called, as we have seen, pseudomutants. When 

 pseudomutations, that is, crossovers, occur in conjunction with 

 the condition of balanced lethals, they closely simulate gen- 

 uine factorial mutations. This is exemplified in the case of 

 De Vries' (Enothera Lamarchiana, which is the product of 

 a crossover supervening upon a situation of balanced lethals. 

 In cases of this kind, the crossover releases hitherto suppressed 

 recessive characters, giving the appearance of real mutation. 

 "The workers with Drosophila," says Davis, "seem inclined 

 to believe that much of the phenomena simulating mutation 

 in their material is in reality the appearance of characters set 

 free by the breaking of lethal adjustments which held the 

 characters latent. Well-known workers have arrived at simi- 

 lar conclusions for (Enothera material and are not content to 



