PHYSIOLOGICAL UNITS 157

is one-sided, sometimes it is a blend. The mother
may look out of one eye, and the father out of another,
or the grandfather may be re-incarnated. By inter-
breeding hybrids pure types may be got, or rever-
sions, or " an epidemic of variations." This is the
problem of the diverse modes of hereditary transmis-
sion, which we know in some cases to be expressible
in a formula, such as Mendel's law or Galton's law,
and for which we can sometimes hazard a hypothetical
physiological interpretation.

Physiological Units.—To each of these three problems
Spencer made a contribution. He started with the
legitimate and fertile hypothesis of " physiological
units"—the ultimate life-bearing elements, inter-
mediate between the chemical molecules and the cell.
Just as the same kinds and even the same number of
atoms compose by different arrangements numerous
quite different chemical molecules, e.g. in the protein-
group, so out of similar molecules diversely grouped
an immense variety of " physiological units " may be
evolved. Out of the same pieces of coloured glass
one may get in the kaleidoscope a very large number
of distinct patterns, so in the course of nature similar
molecules, grouping themselves differently, have
formed a very large number of distinct "physiological
units." The grouping is not merely positional and
static as in the kaleidoscope; it is dynamic and vital.
Since Spencer sketched his idea in 1864 many
biologists have thought of units intermediate be-
tween the chemical molecules and the cell, and
the number of different names which have been
bestowed upon them is extraordinary, each voyager
re-naming his discovery, ignorant of or ignoring thosevolution* scientific and trans-s intrinsic incoherence; worthless as