210 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



by this reduction division contains one set of chromosomes, like the 

 set of fingers on one hand, but unlike the fingers which are perma- 

 nently attached to the hands, the chromosomes are free to change 

 hands so that one germ cell may contain a thumb chromosome from 

 the father, an index from the mother, etc., while the other cell contains 

 corresponding chromosomes from the other parent. This union of 

 parental chromosomes into synaptic pairs and their subsequent sepa- 

 ration in the reduction division exactly parallels the phenomena 

 of Mendelian segregation of characters, and there is no doubt that it 

 is the cause of Mendelian inheritance. 



With these discoveries the foundations were laid for the marvelous 

 developments of cytology in relation to genetics which have character- 

 ized the last 30 years. Thus within the first 5 years of this century 

 were established the Mendelian law of heredity, the mutation theory 

 of evolution, the inability of selection to build up species from fluctua- 

 tions, and the chromosomal mechanisms of sex determination and 

 heredity. 



Ill 



Upon these foundations the study of evolution has advanced with 

 giant strides during the past 25 years. This is especially true of the 

 correlation between mutations, or inherited variations, and the con- 

 stitution of the germ cells. Indeed this correlation has given us for 

 the first time an understanding of the mechanisms of heredity, muta- 

 tion, and evolution. 



Imagine the amazement and incredulity of the naturalists of a 

 former generation, who thought of evolution only as the transforma- 

 tions of developed organisms under the influence of changing en- 

 vironments, if they could learn that today the problems of evolution 

 center largely in the structures and functions of germ cells ! And 

 yet this is strictly and literally true. The germ cells are the only 

 living bonds not only between generations but also between species, 

 and they contain the physical basis not only of heredity but also of 

 evolution. 



In the microscopic chromosomes which are found in the nuclei of 

 all cells and in the ultramicroscopic inheritance units or genes which 

 lie in those chromosomes are found the earliest causes of heredity, 

 sex, mutation, and evolution. In biology as also in physics and 

 chemistry the ultimate causes of phenomena are found not in gross 

 bodies but in their minutest constituents. What molecules and atoms 

 and electrons are to the physicist and chemist, chromosomes and 

 genes are to the biologist. Present problems of evolution are not 

 how one fully developed organism is transmuted into another, for 

 this never happens, but rather how one type of chromosome or gene 

 is transformed into another — not so much the effect of natural selec- 



