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A DIGRESSION 159

tion, only much more energetically dynamic, and (2)
that the big fact of heredity—that like tends to beget
li&e—has its parallel in the way in which a minute
fragment of a crystal can in the appropriate environ-
ment of a solution of the same substance build up
a crystal like the original form from which it was
separated. Germ-cells are potential samples of the
organisation which is expressed in the parent, but
Spencer did not advance to the more distinctively
modern position which recognises that they are
separated off rather from the fertilised ovum which
gave rise to the parent's body than from that body
itself. The parental body is the trustee rather than
the producer of the germ-cells.

A Digression.—Here we must digress a little to
compare Spencer's conception of physiological or
constitutional units with Weismann's conception of
the Germ-Plasm. According to Weismann, there is
in the nuclei of the germ-cells a distinctive physical
basis of inheritance, the germ-plasm. It is the
vehicle of the hereditary qualities, the architectural
substance which enables the germ-cell to build up an
organism \ it has an extremely complex and at the
same time persistent structure. Following a hypo-
thesis of De Vries, he supposed that the readily
stainable nuclear bodies (the chromosomes or idants)
consist of a colony of invisible self-propagating vital
units or biophors, each of which has the power of
expressing in development some particular quality.
He supposed that these biophors are aggregated into
units of a higher order, known as determinants, one
for each structure of the body which is capable of
independent variation. These determinants are sup-



	A DIGRESSION 159 tion, only much more energetically dynamic, and (2) that the big fact of heredity—that like tends to beget li&e—has its parallel in the way in which a minute fragment of a crystal can in the appropriate environ- ment of a solution of the same substance build up a crystal like the original form from which it was separated. Germ-cells are potential samples of the organisation which is expressed in the parent, but Spencer did not advance to the more distinctively modern position which recognises that they are separated off rather from the fertilised ovum which gave rise to the parent's body than from that body itself. The parental body is the trustee rather than the producer of the germ-cells. A Digression.—Here we must digress a little to compare Spencer's conception of physiological or constitutional units with Weismann's conception of the Germ-Plasm. According to Weismann, there is in the nuclei of the germ-cells a distinctive physical basis of inheritance, the germ-plasm. It is the vehicle of the hereditary qualities, the architectural substance which enables the germ-cell to build up an organism \ it has an extremely complex and at the same time persistent structure. Following a hypo- thesis of De Vries, he supposed that the readily stainable nuclear bodies (the chromosomes or idants) consist of a colony of invisible self-propagating vital units or biophors, each of which has the power of expressing in development some particular quality. He supposed that these biophors are aggregated into units of a higher order, known as determinants, one for each structure of the body which is capable of independent variation. These determinants are sup-