A special theory of heredity, pangenesis, has been accepted by many of the Neo-Lamarckian school.  The theory of pangenesis, as propounded by Mr. Darwin, may be very briefly stated as follows:  The cells in all parts of the body are continually throwing off germinal particles, or “gemmules.”  These become scattered through the body, grow, and multiply by division.  On account of mutual attraction they unite in the reproductive glands to form eggs or spermatozoa.  The germ-cells are thus the bearers of heredity because they contain samples, so to speak, of all the organs of the body.

In heredity, according to Weismann’s theory, the egg is the centre of control, the continuous germ-plasm the source of all transmitted changes; according to Darwin’s theory, the body is the source, and the egg is derived in great part at least from it.  If you put to the two the time-honored question, Which is first, the owl or the egg?  Weismann would announce, with emphasis, The egg; Darwin would say, The owl.  One proposition is the converse of the other, and most facts accord almost equally well with both theories.

In any family, devoted for generations to literary or artistic pursuits, the children show, as a rule, an aptitude for such pursuits not manifested by those of other families.  According to the Neo-Lamarckian view, this inherited aptitude is to a certain extent the result of the constant exercise of these faculties through a series of generations.  The active efforts and voluntary disposition of the parents have given an increased predisposition to the child.  “Quite the reverse,” says Weismann, “the increase of an organ in the course of generations does not depend upon the summation of exercise taken during single lives, but upon the summation of more favorable predispositions in the germ.”  “An organism cannot acquire anything unless it already possesses the predisposition to acquire it."[A]

  [Footnote A:  Weismann, Essays, pp. 85 and 171.]

We may accept or deny this last statement, but it is evident that facts like these, and indeed the origin of most or all characteristics involving use or disuse, may be explained almost equally well by either theory.

But as far as the transmission of effects of somatic changes is concerned, if protozoa undergo special modifications under the influence of external conditions, will not the germ-cells undergo special modification under the influence of changes in the somatoplasm which forms their immediate environment?  We must never forget the close relationship between all the cells of the body, and how slight a change in the body or its surroundings may conduce to sterility or fertility.  Such isolation and independence in the body, on the part of the germ-cells, is opposed to all that we know of the organic unity of the body, whose cells have arisen by the differentiation of, and division of labor between, cells primitively alike.  The facts of bud-variation, of changes in the parent stock due to grafting, and others, of which Mr. Darwin has given a summary in the eleventh chapter of the first volume of his “Plants and Animals under Domestication,” have never been adequately explained by Weismann in accordance with his theory.  He has perhaps succeeded in parrying their force by showing that some such explanation is conceivable; they still point strongly against him.