396 



IMPROVEMENT OF THE HUMAN RACE 



are. All three play a part in shaping our destinies. It is self- 

 evident that a handicap such as poor health or lack of education 

 would play an important part in one's success or failure in life. 

 Some men have become great in spite of handicaps, but it was 

 because their heredity was such that they could not be denied 

 success. Let us, then, see if the laws of heredity of which we 

 learned in the last chapter can be applied to man. 



Chromosomes the Bearers of Heredity. — Investigations of 

 heredity have centered, in late years, on the composition and action 



of the chromosomes, those tiny 

 Yelloiv bod/ mm Cray body structures within the nucleus 



mite eye flfl ^^^^/^ of every cell. It has been 



found that they differ in num- 

 ber according to the species of 

 the animal. In a little worm 

 called ascaris and in the fruit 

 fly, there are only four chromo- 

 somes in each germ cell; in 

 the mosquito culex there are 

 six, in the rat sixteen, in the 

 frog twenty-four, in certain 

 crustaceans more than one 

 hundred and fifty, in one 

 spider a hundred and sixty- 

 eight. In some animals, as has been shown, the number differs 

 with sex. Man is believed to have forty-eight. 



Professor Morgan of Columbia University has found, as a result 

 of investigations on the fruit fly (drosophila) , that each chromo- 

 some is actually composed of inheritable stuff that represents 

 unit characters, and that some of these characters are linked to- 

 gether in the same sex. This would explain some of the characters 

 common to only male or female animals. 



It is plain that with forty-eight chromosomes, each of which is 

 probably made up of a large number of determiners of unit char- 

 acters, heredity in man is a very complicated matter at best. 

 But a number of unit characters have been found which are in- 

 heritable according to Mendel's laws, and undoubtedly others will 

 be added as we learn more about the working of these laws. 



Normal mn§\ 



yermilion eye] 

 Miniature mng 



Rudiiuenfary mni\ 

 Forked bristles 

 Complete eye 



Normal iving 



Red eye 



Minldtureiving 



Hudimenfary tvin^ 

 Forked bristles 

 Bar eye 



Section through two chromosomes, show- 

 ing how each may contain several separate 

 unit characters. 



