characters,--the observed
numerical ratio in the following generation follows according to the
law of probability. Thus is explained both the fortuitous element that
differentiates these cases from exact chemical combinations, and the
definite numerical relations that appear in the aggregate of
individuals.

Grandparents AG (white) AB (white) AY (white) CB (black)
| | | |
+---------+ +-----------+
| |
Parents AGAB (white) AYCB (yellow)
| |
+----------------------+
| Observed Calculated
{AGAY}
{ABAY} (White) 81 76
{AGAB}
Offspring ---------------{ABAB}
{
{AGCY} (Yellow) 34 38
{ABCY}
{
{ABCB (Black) 20 19
{AGCB (Gray) 16 19
---- ----
151 152

Now, the point that I desire to emphasize is that one or two very
simple mechanistic assumptions give a luminously clear explanation of
the behavior of the hereditary characters according to Mendel's law,
and at one stroke bring order out of the chaos in which facts of this
kind at first sight seem to be. Not less significant is the fact that
direct microscopical investigation is actually revealing in the
germ-cells a physical mechanism that seems adequate to explain the
disjunction of characters on which Mendel's law depends; and this
mechanism probably gives us also at least a key to the long standing
riddle of the determination and heredity of sex. These phenomena are
therefore becoming intelligible from the mechanistic point of view.
From any other they appear as an insoluble enigma. When such progress
as this is being made, have we not a right to believe that we are
employing a useful working hypothesis?

But let us now turn to a second example that will illustrate a class
of phenomena which have thus far almost wholly eluded all attempts to
explain them. The one that I select is at present one