He compares the plant kingdom to a great, luxurious tree, branching from its very base, whose twigs would represent the present stage of our different species.  Left to itself it would put out a chaos of innumerable branches.  Natural selection, like a gardener, prunes the tree into shape.  Children might imagine that the gardener caused the growth; but the tree would have been broader and have branched more luxuriantly if left to itself.[A]

  [Footnote A:  See Naegeli, “Theorie der Abstammungslehre,” p. 18;
  also pp. 12, 118, 285.]

Every species must vary perpetually.  Now this proposition is apparently not in accord with fact; for some have remained unchanged during immense periods.  And natural selection, by removing the less fit, certainly appears to contribute to progress by raising the average of the species.  The theory seems extreme and one-sided.  And yet it has done great service by calling in question the all-sufficiency of natural selection and the modifying power of environment, and by emphasizing, probably overmuch, the importance of initial inherent tendency, whose value has been entirely neglected by many evolutionists.

Lack of space compels us to leave unnoticed most of the exceedingly valuable suggestions of Naegeli’s brilliant work.

It is still less possible to do any justice in a few words to Weismann’s theory.  Into its various modifications, as it has grown from year to year, we have no time to enter.  And we must confine ourselves to his views of variation and heredity.

In studying protozoa we noticed that they reproduced by fission, each adult individual dividing into two young ones.  There is therefore no old parent left to die.  Natural death does not occur here, only death by violence or unfavorable conditions.  The protozoa are immortal, not in the sense of the endless persistence of the individual, but of the absence of death.  Heredity is here easily comprehensible, for one-half, or less frequently a smaller fraction, of the substance of the parent goes to form the new individual.  There is direct continuity of substance from generation to generation.

But in volvox a change has taken place.  The fertilized egg-cell, formed by the union of egg and spermatozoon, is a single cell, like the individual resulting from the conjugation or fusion of two protozoa.  But in the many-celled individual, which develops out of the fertilized egg, there are two kinds of cells. 1.  There are other egg-cells, like the first, each one of which can, under favorable conditions, develop into a multicellular individual like the parent.  And the germ-cells (eggs and spermatozoa) of volvox are immortal like the protozoa.  But, 2, there are nutritive, somatic cells, which nourish and transport the germ-cells, and after their discharge die.  These somatic cells, being mortal, differ altogether from the germ-cells and the protozoa.  The protoplasm must differ in chemical, or molecular, or other structure in the two cases, and we distinguish the germ-plasm of the germ-cells, resembling in certain respects Naegeli’s idioplasm, from somatoplasm, which performs most of the functions of the cell.  The somatoplasm arises from, and hence must be regarded as a modification of, the germ-plasm.  The germ-plasm can increase indefinitely in the lapse of generations, increase of the somatoplasm is limited.