122 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



mutant races will convince one that the viability of a mutant race is, 

 on the average, inversely proportional to the degree of its difiference 

 from the parent race. In other words, the greater the change pro- 

 duced by the mutation the more likely it is to interfere with the proper 

 functioning of the organism. An organism is an extremely delicately 

 adjusted mechanism, and any random change in it might be expected 

 to decrease the efficiency of the whole. Furthermore, the greater 

 the change the greater is the damage that is likely to result. It fol- 

 lows, then, from actual observation as well as from theoretical con- 

 siderations, that mutations which bring about slight changes are least 

 likely to be harmful and therefore are most likely to become incor- 

 porated into the race. This gives an explanation for the observed 

 fact that specific differences are usually slight ones. The reason that 

 the observed mutational differences are often of much greater degree 

 is very simple; w^hen such changes do occur they are easily discovered 

 and are convenient to work with, so that they are artificially selected 

 and perpetuated even if their viability or productivity is inferior to 

 that of the parent race. 



That species commonly differ in more respects than do mutant 

 races must mean that they differ in more inherited factors than do 

 mutant races. Mutant races are usually known to differ significantly 

 from the parent race in only one or a very few genes. That species 

 differ from each other in many genes is, in the case of Drosophila or 

 any form in which fertile hybrids are not known, only an inference.* 

 Such a situation would, however, be pretty certain to arise as a result 

 of long-continued isolation. In the case of the species of Drosophila 

 that have been studied, interspecific sterility constitutes an effective 

 mechanism for bringing about isolation. As to the origin of the inter- 

 specific sterility itself, we can only speculate until we know more about 

 the mechanism whereby such sterility is now brought about. 



Species, then, differ from each other in many genes. The differ- 

 ences, though numerous, are such that each produces only a slight 

 effect on the organism. These differences are of the same kind as are 

 the mutational differences, and may be supposed to have arisen by 

 mutation. 



The picture of evolution that this analysis leads to is in effect not 

 very different from that which Darwin drew. Species change gradu- 

 ally, by the slow accumulation of numerous slight mutational differ- 

 ences. The possibility of a sudden change of considerable degree is 

 always present, but wdll not usually be realized, because such a change 

 will generally give rise to an imperfectly adjusted organism. 



* This point has, however, been demonstrated in the case of certain species hybrids among 

 plants. Compare Baur (1919, Zeits. ind. Abst. Vererb., 21, 48-52) and Lotsy (1912, Zeits. ind. 

 Abst. Vererb., 8, 325-333) on Antirrhinum; Wichler (1913, Zeits. ind. Abst. Vererb., 10, 177- 

 232) on Dianthus; Kristo£ferson (1914, Botan. Notis., pp. 25-31, abstract in Zeits. ind. Abst. 

 Vererb., 14, 34) on Viola; van der Stok (1910, Teysmannia, 21, 47-59, abstract in Zeits. ind. 

 Abst. Vererb., 4, 153) on corn-teosinte hybrids; East (1916, Genetics, 1, 311-333) on Nicotiana. 



