far-reaching extent; it appears, however, to be the only right way by
which we can finally reach the solution of a question the importance of
which cannot be over-estimated in connection with the history of the
evolution of organic forms."

He tells us also why he selected peas for his work:

"The selection of the plant group which shall serve for experiments of
this kind must be made with all possible care if it be desired to avoid
from the outset every risk of questionable results."

"The experimental plants must necessarily

1. Possess constant differentiating characters.

2. The hybrids of such plants must, during the flowering period, be
protected from the influence of all foreign pollen, or be easily
capable of such protection."

Why do biologists throughout the world to-day agree that Mendel's discovery
is one of first rank?

A great deal might be said in this connection. What is essential may be
said in a few words. Biology had been, and is still, largely a descriptive
and speculative science. _Mendel showed by experimental proof that heredity
could be explained by a simple mechanism. His discovery has been
exceedingly fruitful._

Science begins with naive, often mystic conceptions of its problems. It
reaches its goal whenever it can replace its early guessing by verifiable
hypotheses and predictable results. This is what Mendel's law did for
heredity.

MENDEL'S FIRST DISCOVERY--SEGREGATION

[Illustration: FIG. 13. Diagram illustrating a cross between a red (dark)
and a white variety of four o'clock (Mirabilis jalapa).]

Let us turn to the demonstration of his first law--the law of segregation.
The first case I choose is not the one given by Mendel but one worked out
later by Correns. If the common garden plant called four o'clock (Mirabilis
jalapa) with red flowers is crossed to one having white flowers, the
offspring are pink (fig. 13). The hybrid, then, is intermediate in the
color of its flowers between the two parents. If these hybrids are inbred
the offspring are white, pink and red, in the proportion of 1:2:1. All of
these had the same ancestry, yet they are of three different kinds. If we
did not know their history it would be quite impossible to state what the
ancestry of the white or of the red had been, for they might just as well
have come from pure white and pure red ancestors respectively as to have
emerged from the pink hybrids. Moreover,