PHENOMENA OF INHERITANCE 3 J 5 



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 pro- 

 duces 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 espe- 

 cially 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 in- 

 numerable 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 stat- 

 ure 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 excep- 

 tions. 



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 peculiari- 

 ties 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 characteris- 

 tic 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 



