32 POPULAR SCIENCE MONTHLY. 



THE PHYSICAL BASIS OF HEREDITY.* 



By Professor CARL H. EIGENMANN, 



INDIANA UNIVERSITY. 



i TT E is a chip of the old block ' is the popular expression used in 

 J — *- applying the best known general law of heredity that ' like be- 

 gets like ' to a particular case. But in saying so we state but half the 

 truth. The chip is like the block and not like the block. Each in- 

 dividual is unique; no two were ever cast in the same mold. 



There are always two phenomena associated in the development of 

 a new individual. One is the repetition in the offspring of characters 

 possessed by his ancestors, either near or remote. The second is the 

 formation of new characters which have never appeared before in any 

 individual. I shall confine myself to the first of these phenomena. 



What characters has any individual inherited from his ancestors 

 and what ones are new to him is always a question of the first consid- 

 eration in a study of heredity. The consideration of which of these 

 he may transmit to his offspring naturally follows. 



He has always inherited and always transmits those characters that 

 distinguish his species, race or family from other species, races or 

 families. A backbone, four limbs and hairiness are always inherited 

 and transmitted by a mammal, as backbone, four limbs and feathers 

 are always transmitted by a bird. The erect position, peculiarities of 

 hand and foot and those other features which together make a man 

 are always inherited and always transmitted. The inheritance and 

 transmission of the racial characters of the Jews distinguish them from 

 all the various peoples with which they are found. The same is true 

 of the Chinese, Indians, Negroes and, to a less degree, of the less pure 

 races of Teutons and Anglo-Saxons. 



Aside from characters that are always inherited and transmitted, 

 there are groups of characters that may have been inherited and that 

 are transmissible, but that are not necessarily transmitted. It is the 

 peculiar combination of some of these characters that constitutes family 

 traits. These transmissible, but not necessarily transmitted, char- 

 acters may be anatomical and range from the height of the individual 

 to such minute details as moles, a few long hairs in the eyebrows or 

 even smaller details. 



They may be physiological. Longevity is transmissible; so are a 



* Photographic illustrations by D. W. Dennis. 



