488 GENETICS 



tion to the pigment on the back of the iris, there may be pigment 

 on the front of it. This produces eye-colors from hazel to dark 

 brown. The presence of pigment on both sides of the iris is dom- 

 inant to the absence of pigment from the front of the iris, that is, 

 to its presence on the back only. Therefore, brown eyes are 

 dominant to blue eyes. In the rare condition known as albinism, 

 pigment is absent from both sides of the iris, and the blood-vessels 

 of the iris give the eye its pink color. Such a condition is recessive 

 to blue eye-color. Curly hair is dominant to straight hair. In 

 color-blindness, which is the result of an inherited defect of the 

 retina, and in haemophilia, a defect of the blood that prevents its 

 clotting, the genes are sex-Knked. Mental as well as physical 

 characteristics appear to be inherited. The tracing of families 

 like the Jukes, the Kallikaks, and the Edwards indicates that 

 mental qualities leading to degeneracy, crime, and pauperism, as 

 well as those yielding leadership in all social fields, may be 

 inherited according to Mendelian principles. 



The environment in which a particular gene-complex develops 

 may limit and obscure its possibilities, as we have seen. Thus a 

 given combination of genes in a human individual may produce a 

 better adult in a favorable environment than it could in an unfavor- 

 able one. On the other hand, a good combination of genes will 

 give rise to a better individual in a particular environment than 

 will a poor set of genes. Training is another element that is very 

 important in the unfolding of human potentialities. An inferior 

 inheritance with superior training may result in an individual bette'* 

 fitted for society than a good inheritance with no training. But 

 no amount of training can produce anything for which the inher- 

 ited capacities are not present, nor can the best of environment 

 implant qualities that are lacking in the germplasm. In other 

 words, heredity limits very definitely the possibilities of training 

 and environment (c/. Fig. 232, p. 445). Eugenics is that particular 

 branch of Apphed Genetics which deals with the improvement of 

 the mental and physical characteristics of future generations of the 

 human race. Its problems are: first, extension of our knowledge 

 concerning the facts of human heredity; and, second, the educa- 

 tion of the pubHc for appreciation of the meaning and application 

 of this knowledge. For the methods of ehminating inferior germ- 

 plasm and perpetuating superior genetic factors, human society 

 must turn to Genetics. 



