And, finally, I want to urge on your attention a question that we are to
consider in more detail in the last lecture. Evolution of wild species
appears to have taken place by modifying and improving bit by bit the
structures and habits that the animal or plant already possessed. We have
seen that there are thirty mutant factors at least that have an influence
on eye color, and it is probable that there are at least as many normal
factors that are involved in the production of the red eye of the wild fly.

Evolution from this point of view has consisted largely in introducing new
factors that influence characters already present in the animal or plant.

Such a view gives us a somewhat different picture of the process of
evolution from the old idea of a ferocious struggle between the individuals
of a species with the survival of the fittest and the annihilation of the
less fit. Evolution assumes a more peaceful aspect. New and advantageous
characters survive by incorporating themselves into the race, improving it
and opening to it new opportunities. In other words, the emphasis may be
placed less on the competition between the individuals of a species
(because the destruction of the less fit does not _in itself_ lead to
anything that is new) than on the appearance of new characters and
modifications of old characters that become incorporated in the species,
for on these depends the evolution of the race.

* * * * *

CHAPTER III

THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM

The discovery that Mendel made with edible peas concerning heredity has
been found to apply everywhere throughout the plant and animal kingdoms--to
flowering plants, to insects, snails, crustacea, fishes, amphibians, birds,
and mammals (including man).

There must be something that these widely separated groups of plants and
animals have in common--some simple mechanism perhaps--to give such
definite and orderly series of results. There is, in fact, a mechanism,
possessed alike by animals and plants, that fulfills every requirement of
Mendel's principles.

THE CELLULAR BASIS OF ORGANIC EVOLUTION AND HEREDITY

In order to appreciate the full force of the evidence, let me first pass
rapidly in review a few familiar, historical facts, that preceded the
discovery of the mechanism in question.

[Illustration: FIG. 45. Typical cell showing the cell wall, the protoplasm
(with its contain