Page:Popular Science Monthly Volume 61.djvu/38

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32
THE POPULAR SCIENCE MONTHLY.
THE PHYSICAL BASIS OF HEREDITY.[1]
By Professor CARL H. EIGENMANN,
INDIANA UNIVERSITY.

'HE is a chip of the old block' is the popular expression used in applying the best known general law of heredity that 'like begets 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 individual 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 consideration 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, characters 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

  1. Photographic illustrations by D. W. Dennis.