In the ancestry of the one is to be
found no single over a long series of years; in the ancestry of the
other nothing but singles occurred. The creature of given constitution
may often be built up in many ways, but once formed it will behave like
others of the same constitution."[29]
[Footnote 29: Encyclopaedia Britannica, Vol. XVIII, p. 119.]
IV
Vanished at last are the old theories of gradual changes in species
perpetuated and accumulated by natural selection until at last wholly
new forms have in this way been produced. True variations are now seen
to be confined within well-marked and rather narrow limits, within which
ordinary variations may occur, perhaps induced by environment. These
fluctuating variations grade off into one another on all sides, and
their differences _can_ be plotted on a frequency curve; but the very
important thing for us to remember is that these fluctuating variations
_cannot be transmitted._ Beyond these fluctuating variations come the
unit characters or factors, which are distinct from each other, or
"discontinuous," to use the technical term, and which therefore _cannot
be plotted on a frequency curve_. These factors are not modified in the
least by the environment, and their peculiarities are faithfully
transmitted in heredity with all the precision of chemical law.
Pages:
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100