254 THE SCIENTIFIC MONTHLY
many of you. In this study attention is concentrated not upon the influence which one parent as a whole exerts upon a descendant, but upon the transmission of particular characteristics. The characteristics to which attention is paid are those in which the two parents differ sharply. They are contrasting characters like blackness and whiteness of fur in the rabbit, tallness and dwarf ness of the pea vine, roughness and smoothness of coat in the guinea pig.
The conclusion of fundamental importance is that such characters do not blend in the descendants, but are passed on from generation to generation in their original distinctness. The characters, Mendelian or unit-characters as they are called, may be obvious or latent In the familiar case of rabbit breeding, when a black and a white rabbit are bred from, the offspring are all black, but whiteness is latent in some, for if the black offspring are interbred, a certain proportion of white rabbits will appear among the grandchildren.
A point of importance is that the Mendelian characters of an ances- tor behave in heredity independentiy of one another in such wise that new combinations may be made. Thus, if a dark, smooth guinea pig be bred to a white rough guinea pig, and the offspring be interbred, the grandchildren will be of four kinds, with respect, that is, to the qualities darkness and whiteness, smoothness and roughness (W. E. Castie). Some will be like the grandfather and some like the grandmother. But there will be other grandchildren like neither of the grandparents. In these a grandfather feature is combined with a grandmother feature, and so we get dark rough and white smooth pigs.
Thus qualities which exist apart from one another in separate organ- isms may be combined in one and the same individual, and new breeds be created. In such new breeds it is apparent that new qualities are not created. What is created is a new combination. This is heritable and therefore marks off the breed from others. Hybridization here, then, originates heritable differences between organisms. It may be added that the independent behavior of Mendelian characters in heredity is not necessarily equal throughout a long series of characters. In other words, characters sometimes, perhaps always, tend to reappear in groups. This important fact has been especially brought out by recent work on the heredity of the littie fruit-fly, Drosophila (T. H. Morgan).
In a loose and general way it has always been known that new com- binations of characters occur in organisms bred from two parents. In this connection G-oethe's verses have often been quoted by Haeckel and others:
From father I get my height
And my earnestness;
From mother dear mj gladness of nature
And delight in romancing.^
1 * ' Vom Vater hab ' ich die Statur, ' ' etc.
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