208 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



However, mutations are always inherited, that is, they represent 

 changes in the germ plasm, whereas changes which first occur in 

 developed organisms are not inherited and are called fluctuations. 

 This is indeed the chief distinction between the old evolution of 

 Lamarck and Darwin and the new of Weismann and De Vries; in 

 the old, attention was fixed upon the developed organism and evolu- 

 tionary changes were supposed to be first made in the adult and 

 then by some mysterious process to be transferred to the germ cells ; 

 in the newer views of evolution changes are first wrought in the 

 germ cells and only later appear in the developed organism. 



In 1903 Johannsen found that by continued breeding and isolation 

 of self-fertilized beans he could isolate from a so-called pure garden 

 variety, 19 different " pure lines " and that further selection within 

 any one of these lines was without effect. Other similar results in a 

 large variety of plants and animals led to the conclusion that neither 

 artificial nor natural selection could have the effect, which Darwin 

 had postulated, in building up a species from small variations. By 

 some this was hailed as the " death of Darwinism ", or natural selec- 

 tion, as a factor in evolution, but it was soon seen to apply only to 

 fluctuations and not to mutations. It is true that selection cannot 

 create mutations but it can act upon mutations that are offered and 

 recent work in the field of genetics has shown that it is a potent 

 factor in evolution. 



Almost coincidentally with the rediscovery of Mendelism and the 

 establishment of the mutation theory came the discovery of the cellu- 

 lar basis of these phenomena in the germ cells. The work of certain 

 European biologists had previously furnished evidence that the in- 

 heritance material is located in the nuclei of the germ cells and 

 chiefly if not entirely in certain threads, called chromosomes, that 

 are found in those nuclei. When egg and sperm unite in fertilization 

 their chromosomes commingle but retain their individual identity 

 and in the repeated divisions of the fertilized egg, which lead to the 

 developed animal or plant, every chromosome in every nucleus splits 

 lengthwise and its halves separate, going into the two daughter cells; 

 this is repeated at every cell division until every cell of the developed 

 organism has half of its chromosomes from the egg and half from 

 the sperm. Finally when this adult organism in turn forms eggs or 

 spermatozoa the number of chromosomes in these sex cells is reduced 

 to half those present in all other cells. And when the chromosomes 

 of egg and sperm unite in fertilization, the full number is again re- 

 stored. Since, on the average, organisms inherit as many traits from 

 one parent as from the other, and, since they receive an equal number 

 of chromosomes from each parent, it seemed highly probable that 

 the chromosomes contained the inheritance material, but at the begin- 



