780 A TEXTBOOK OF PHYSIOLOGY 



breeding of the mixed impure dominant type again yields 25 per 

 cent. D, 50 per cent. D(R), and 25 per cent. R. This may be 

 tabulated as follows: 



D R 



\/ 

 D(R) 



ID 2D(R) 1R 



I 



D 1D+2D(R) + 1R R 



Many experiments to prove this law have been made both upon 

 plants (peas, beans, maize, wheat, stocks, etc.) and animals (mice, 

 rats, poultry, canaries, moths). In peas, for example, it is claimed 

 that tall stems, yellow cotyledons, brown-skinned seeds, and round 

 seeds are dominant characters; while dwarf stems, green cotyledons, 

 white seeds, and wrinkled seeds are recessive characters. Among 

 animals, dominant characters are short hair in rabbits, hornlessness 

 in cattle, crest in poultry, brown eyes in man, etc. ; recessive are long 

 hair in rabbits, horns in cattle, absence of crest in poultry, grey and 

 blue eyes in man. The explanation given of this law is that these 

 characters, dominant and recessive, are segregated in two different 

 sets of germ cells. 



Although the law derives support from many characters, such as 

 those mentioned above, and from various hereditary diseases and mal- 

 formations of the human race, such as brachydactyly, it does not explain 

 all hereditary phenomena. The cross-breds of a white and black, when 

 intermarrying, do not produce 25 per cent, pure white, 50 per cent, 

 mixed, 25 per cent, black, but offspring of varying degrees of duskiness. 



Following Mendelism, the modern school of eugenists, bent upon 

 the " improvement " of the race, maintain that race improvement 

 is solely a matter of breeding from good stock. This may undoubtedly 

 lead to physical fitness, but it is very questionable as to whether it is 

 the only way. It is also a difficult question to determine at what 

 fitness we are to aim. It is well known that " genius " cannot be 

 made to breed true. A genius in a family is a " spontaneous " varia- 

 tion, as much as a child with six fingers. How do such " spontaneous " 

 variations arise ? 



Darwin stated that new varieties of species arose by the cumulative 

 effect of natural selection upon small fluctuating variations. It is of 

 first-rate importance to ascertain how new varieties, healthy, intel- 

 ligent, honest, diseased, feeble-minded, and criminal, arise. Can the 

 parents' drunkenness, for example, affect the germ plasm ? At 

 present, " eugenic " principles seem of little help. Darwin confessed: 

 '' Our ignorance of the laws of variation is profound." It still is. 



It has recently been suggested, as the result of observations in 

 the vegetable kingdom, that species arise from one another by dis- 



