Page:Popular Science Monthly Volume 85.djvu/319

Rh otherwise there would be no constant characteristics of these groups and no possibility of classifying organisms. The chief characters of every living thing are unalterably fixed by heredity. Men do not gather grapes of thorns nor figs of thistles. Every living thing produces offspring after its own kind. Men, horses, cattle; birds, reptiles, fishes; insects, mollusks, worms; polyps, sponges, microorganisms—all of the million known species of animals and plants differ from one another because of inherited peculiarities—because they have come from different kinds of germ cells.

2. Individual Characters.—Many characters which are peculiar to certain individuals are known to be inherited, and in general use the word inheritance refers to the repetition in successive generations of such individual peculiarities. Among such individual characters are the following:

(a) Morphological Features.—Hereditary resemblances are especially recognizable in the gross and minute anatomy of every organism in the form, structure, location, size, color, etc., of each and every part. The number of such individual peculiarities which are inherited is innumerable and only a few of the more striking—of the greatest and smallest of these can be mentioned.

It is a matter of common knowledge that unusually great or small stature runs in certain families, and Galton developed a formula for determining the approximate stature of children from the known stature of the parents and from the mean stature of the race. However his statistical and mathematical formulas give only general or average results, from which, there are many individual departures and exceptions.

In the same way the color of the skin, the color and form of hair, and the color of eyes are, in general, like those of one or more of the parents or grandparents. We all know that certain facial features such as the shape and size of eyes, nose, mouth and chin are generally characteristic of certain families.

But the inheritance of anatomical features extends to much more minute characters than those just mentioned. In certain families a few hairs in the eyebrows are longer than the others; or there may be patches of parti-colored hair over the scalp; or dimples in the cheek, chin, or other parts of the skin may occur; and these trifling peculiarities are inherited with all the tenacity shown in the transmission of more important characters. Johannsen has found races of beans in which the average weight of individual seeds differed only by.02 to .03 gram, and yet these minute differences in weight were characteristic of each race and were of course inherited. Jennings has found races of paramecium which show hereditary differences of.005 mm. in length (Fig. 46). Nettleship says that the lens of the human eye weighs only about 175 milligrams, or about one three millionth part