NUTRITION AND REPRODUCTION 129

modifications, there must be some other reason why
the circuitous process of sexual reproduction has been
preferred. We believe the answer to be that sexual
reproduction is an adaptive process securing the
benefits of amphimixis, for in amphimixis and in the
changes preparatory to it, there is an important source
of variation. In one of his essays Weismann wrote as
follows:—

"Sexual reproduction is well known to consist in the
fusion of two contrasted reproductive ceils, or perhaps even
in the fusion of their nuclei alone. These reproductive
cells contain the germinal material or germ-plasm, and this
again, in its specific molecular structure, is the bearer of the
hereditary tendencies of the organisms from which the re-
productive cells originate. Thus in sexual reproduction two
hereditary tendencies are in a sense intermingled. In this
mingling, I see the cause of the hereditary individual
characteristics; and in the production of these characters,
the task of sexual reproduction. It has to supply the material
for the individual differences from which selection produces
new species."

When we inquire into the reasons for the occurrence of a
process such as sexual reproduction, there are four different
questions which may be put: (i) We may inquire into
the historical evolution of the process, so far as that can be
legitimately imagined or inferred from still persistent grades.
(2) We may try to discover what factors may have operated
in the course of evolution in raising the process from one step
of differentiation to another. (3) We may also try to show
how the process is justified by its advantages either self-
regarding or species-maintaining. (4) We may inquire in-
to the physiological sequences in the internal economy of the
individual organism which lead up to the process in question.
There is no doubt always an immediate necessity for the
occurrence of an organic process, but we are in many cases
quite unable at present to do more than describe the series of
events without understanding their causal nexus. The reason
for this is apparent, since the organism is much more than aich become