September 25, 1903.] 



SCIENCE. 



397 



study of heredity perhaps the greatest, was 

 made by Gregor Mendel, an Austrian monk, 

 in the garden of his cloister, some forty 

 years ago. The discovery was announced 

 in the proceedings of a fairly well-known 

 scieutifie society, but seems to have at- 

 tracted little attention and to have been 

 soon forgotten. The Darwinian theory 

 then occupied the center of the scientific 

 stage and Mendel's brilliant discovery was 

 all but unnoticed for a third of a cen- 

 tury. Meanwhile the discussion aroused by 

 Weismann's germ-plasm theory, in partic- 

 ular the idea of the non-inheritance of ac- 

 quired charactere, had put the .scientific 

 public into a more receptive frame of 

 mind. Mendel's law was rediscovered in- 

 dependently by three difi'erent botanists 

 engaged in the .study of plant-hybrids— de 

 Vries, Correns and Tschermak— in the 

 year 1900. It remained, however, for a 

 zoologist, Bate.son, two years later, to point 

 out the full importance and the wide appli- 

 cability of the law. Since then the Men- 

 delian discoveries have attracted the atten- 

 tion of biologists generally. Accordingly a 

 brief .statement of their underlying princi- 

 ples may not be without interest to others 

 also. 



1. The Law of Dominance.— When 

 mating occurs between two animals or 

 plants differing in some character, the off- 

 spring frequently all exhibit the character 

 of one parent only, in which case that 

 character is said to be 'dominant.' Thus, 

 when white mice are crossed with gray 

 mice, all the offspring are gray, that color 

 character being dominant. The character 

 which is not seen in the immediate off- 

 spring is called 'recessive,' for though un- 

 seen it is still present in the young, as we 

 shall see. White, in the instance given, is 

 the recessive character. The principle of 

 heredity just stated may be called the law 

 of dominance. The first instance of it dis- 

 covered by Mendel related to the cotj'le- 



don-color in peas obtained bj" cro.ssiug dif- 

 ferent garden varieties. Yellow color of 

 cotyledons was found to be dominant over 

 green ; likewise, round smooth form of seed 

 was found to . be dominant over angular, 

 wrinkled form ; and violet color of blos- 

 soms, over white color. Other illustrations 

 might be mentioned both among animals 

 and among plants, but these will suffice. 



2. Peculiar Hybrid Forms. — The law of 

 dominance is not of universal applicabil- 

 ity; Mendel does not so declare, though 

 some of his critics have thus interpreted 

 him. In many cases the cross-bred off- 

 .spring possess a character intermediate 

 between those of the parents. This Mendel 

 found to be true when varieties of peas 

 differing in height were crossed. 



Again, the cross-breds may possess what 

 appears to be an intensification of the char- 

 acter of -one parent, as when in crossing 

 dwarf with tall peas the hybrid plant is 

 taller than either parent, or as when, in 

 crossing a brown-seeded with a white- 

 seeded variety of bean, the offspring bear 

 beans of a darker brown than those of the 

 brown-seeded parent. 



Thirdly, the cross-bred may have a char- 

 acter entirelj' dift'erent from that of either 

 parent. Thus a cross betw-een spotted, 

 black-and-white mice, and albino mice, 

 produces commonly mice entirely gray in 

 color, like the house-mouse. Again, in 

 crossing beans, a variety having yellowish- 

 brown seeds crossed with a white-seeded 

 variety yields sometimes black-mottled 

 seed, a character possessed by neither 

 parent. 



These three conditions may be grouped 

 together by saying— the hybrid often pos- 

 sesses a character of its own, instead of the 

 pure character of one parent, as is true in 

 cases of complete dominance. The hybrid 

 character may approximate that of one 

 parent or the other, or it may be different 

 from both. There is no way of predicting 



