Page:Popular Science Monthly Volume 63.djvu/226

222 chromosomes this does not prove that the chromosomes cause the other differences, even though the differences of the chromosomes interfere with the conjugation of the reproductive cells and thus prevent the hybridization of the plants. Species or varieties seldom, if ever, differ by single characters or at one stage merely, and there is no known reason why related species should not diverge in their single-celled condition as well as at any later period. It is rapidly becoming apparent that the internal organs and functions of cells are as diverse as those of embryos and adult organisms, and as much in need of a general evolutionary explanation,

The notion that heredity, variation or other phases of evolution are the functions of special organs or mechanisms of cells, has no ascertained basis of fact, and is but an inference from the traditional evolutionary errors that species are normally constant or stable, and that developmental changes are the results of external influences. To move a stationary organism some sort of 'hereditary mechanism' would be needed to bring about the inheritance of characters 'acquired' from the environment, but if we consider that the individuals of a species are normally diverse, and that the species as a whole is normally in motion, a 'hereditary mechanism' becomes quite superfluous, or may be identified with the organism itself, whether in a unicellular or a polycellular stage.

Heredity is the term under which we allude to the fact that organisms exist in series of similar individuals; we have as yet no warrant for holding that it is special 'force' or agency. Crystals of the same substance are thought of as repeatedly taking the same form because of certain properties of matter, not because of a special crystallizing mechanism. The analogy of crystals is, of course, quite inadequate for biological purposes, but we need not reject it entirely, since for all purposes of expression heredity is a general property of living things, and with these there is even less reason than with crystals to seek a cause in the function of a special organ.

Inorganic elements and compounds are homogeneous and similar in all masses or parts; but diversity is the rule among organisms, no two of which are exact duplicates. The idea of a heredity which maintains identity of structure or form represents no fact in nature. The necessity of continued readjustment is general in life, and is not confined, even in complex organisms, to preliminary stages or to reproductive cells. The individual is not constant nor permanent, but has its own cycle of growth, reproduction and decline, accompanied by continuous changes in all parts of the bodily form and structure.