EVOLUTION CONKLIN 211 



tion in eliminating certain adult forms and preserving others, 

 although this does occur, as its much gi-eater effect in eliminating 

 certain types of embryos, germ cells, and genotypes. 



No longer do biologists discus how adult characters can be 

 crowded back into the egg, nor how characters acquired by an adult 

 can be inherited, for they are almost unanimously agreed that these 

 things never happen, but rather they investigate how changes in 

 chromosomes and genes are produced and how these give rise to 

 changes in the developed organism. This revolution in the study of 

 evolution had its remote beginnings in the nineteenth century, but its 

 most significant results are confined entirely to the present century, 

 most of them to the past 20 years. 



It is impossible in the brief time at my disposal to deal with all 

 of the significant advances of these recent years in the study of 

 evolution and I must of necessity select only a few for presentation. 

 Perhaps the most significant of these discoveries relate to the causes 

 of mutations. In general it may be said that they are caused (1) 

 by changes in the numbers and associations of whole chromosomes, 

 (2) changes in the composition of individual chromosomes, and (3) 

 changes in the position and constitution of the genes themselves. 

 De Vries did not attempt to trace the mutations of his evening 

 primroses to the chromosomes, but other younger persons, many of 

 them Americans, did this, and they found that the original form, 

 Oenothera lavia7'ckmna, has 14 chromosomes, whereas there are 15 

 chromosomes in 7 different mutants — among them 0. lata, 0. albida, 

 and O. scintillans, while in O. gigas there are 28 and in O. semigiga^ 

 21. Since there are typically 7 chromosomes in each of the male 

 and female sex cells of O. laTuarckiana, it seemed probable that these 

 mutants were produced from sex cells, some of which had more than 

 7 chromosomes. It sometimes happens that a synaptic pair of 

 chromosomes fails to separate (nondisjunction) in the reduction 

 division in which case 8 chromosomes go into one sex cell and 6 

 into the other. If then a sex cell having 8 chromosomes unites with 

 one having the normal number 7, a form with 15 chromosomes results 

 and if this additional chromosome is from a different synaptic pair 

 in different cases it would account for the differences in those 

 mutants each of which has 15 chromosomes. Likewise if all the 

 synaptic pairs fail to separate, it leads to the production of a sex 

 cell having 14 chromosomes, and if such a cell unites with a normal 

 sex cell with 7, it produces the mutant semigigas with 21 chromo- 

 somes. If both male and female sex cells fail to undergo reduction 

 each would contain 14 chromosomes, and if two such should unite 

 it would produce the mutant gigas with 28 chromosomes. There 

 are other peculiar modifications of the chromosomes of Oenothera, 



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