Page:Popular Science Monthly Volume 85.djvu/432

428 E a factor for the extension of black or brown but not of yellow.

Plate found that all of these factors except the last, E, are also involved in the production of the coat colors of mice. Baur has recognized more than twenty different factors for the color and form of flowers in the snap-dragon, Antirrhinum.

These factors are probably complex chemical substances which preserve their individuality in various combinations, just as groups of atoms or radicals do in chemical reactions; they may be dropped out or added, substituted or transposed, just as chemical radicals may be in chemical compounds. To this extent they maintain continuity and independence, but they are not absolutely independent, for they react upon one another as well as to environmental changes, so that the characters of the developed organism are the results of all these reactions and interactions.

It is obvious that there must be things in germ cells which correspond to the inheritance factors; furthermore, these things must be material particles even though they be only atoms or molecules and their combinations or dissociations. And yet there are many students of the phenomena of heredity who know little about germ cells and to whom all parts of a cell are hypothetical structures, to whom "chromosomes are articles of faith," and who protest rather violently against any attempt to find the factors of inheritance in any of the structures of the germ cells. And yet it is perfectly evident that if there are inheritance units they must exist in the germ cells as discrete particles, even if they are only molecules, by whose associations or dissociations in response to intrinsic or extrinsic conditions the various characters of the developed organism arise. It is certainly legitimate to ask what the germinal elements are which correspond to inheritance factors.

There was a time when the cell was the ultima thule of biological analysis and when the contents of cells were supposed to be "perfectly homogeneous, diaphanous, structureless slime." Then the nucleus was discovered within the cell, then the chromosomes within the nucleus, then the chromomeres within the chromosomes, and there is no reason to suppose that organization ceases with the powers of our present microscope. With every improvement of the microscope and of microscopical technique, structures have been found in cells which were undreamed of before, and it is not probable that the end has been reached in this regard. We know that cells contain nuclei and chromosomes and chromomeres, centrosomes and plastosomes and microsomes, and we know that some of these parts differ in function as well as in structure. And there is no reason to doubt that if we had sufficiently powerful microscopes we should find still smaller and smaller units until we came at last to molecules and atoms.