598 



COLLEGE ZOOLOGY 



Figure 423. Chang and Eng, the most famous 

 pair of Siamese (conjoined) twins. The photograph 

 clearly shows the band of flesh connecting them, 

 which is evidence that conjoined twins are due to 

 an incomplete separation of the embryo. They are 

 always of the same sex. Chang and Eng were both 

 married and said to have had 22 children. (Photo 

 courtesy of F.H. Meserve.) 



characteristics to its offspring to the exclu- 

 sion of those of the other parent. Males are 

 supposed to be particularly prepotent, but 

 this is no doubt due to the fact that animal 

 breeders pay more attention to males than 

 to females for breeding purposes. 



Inheritance of so-called 

 acquired traits (characters) 



Some of the traits selected by animal and 

 plant breeders arise during the life of the 

 individual as the result of environmental 

 influences. Whether or not these acquired 

 traits are inherited is a problem that has in- 

 terested biologists for many years. None of 

 the experiments designed to test the in- 

 heritance of acquired traits has given posi- 

 tive results, although there are a few cases 

 that may be in doubt. It may safely be said 

 that as yet there is no good evidence for the 



inheritance of acquired traits. This does not 

 mean that the environment has no influence; 

 on the contrary, development of the traits 

 of an individual depends on both genetic 

 and environmental components (Fig. 424). 



Eugenics and euthenics 



Eugenics is the science concerned with 

 the application of the principles of genetics 

 to the improvement of the human species. 

 There is every reason to believe that the 

 principles of heredity are as applicable to 

 man as to other organisms, but the practical 

 difficulties of actually applying these prin- 

 ciples, at least to the conservation of the 

 human species, are very great. 



The study of human genetics is difficult 

 for several reasons: (1) the impracticability 

 of experimental breeding, (2) the small 

 number of offspring in a family, and (3) 

 the relatively slow breeding of man. Despite 

 all these handicaps, significant achievements 

 are being made in this field as is evident 

 from the fact that in the last 15 years, cen- 

 ters for counseling on heredity have been 

 established as a practical way of giving in- 

 formation to people with genetic problems. 



Physicians are realizing more and more 

 that human genetics has an important bear- 

 ing on clinical problems, and relatively rapid 

 progress is being made in the accumulation 

 of information, which, when applied, proves 

 helpful in the prevention and diagnosis of 

 disease. The application of genetics to med- 

 ical problems is a new science called medical 

 genetics; it is making contributions to 

 eugenics, for its practical application here 

 seems more advanced than in some related 

 fields. 



Efforts are now being made to collect data 

 which can be used in determining which 

 human traits are hereditary, how these are 

 inherited, and to what degree. Such informa- 

 tion may aid in the solution of some of the 

 socio-economic problems with which so- 

 ciety is confronted. 



Certain studies show that people with 



