124 HERBERT SPENCER

as long as the forces which result in growth are greatly
in excess of the antagonistic forces. Conversely the
recurrence of sexual reproduction occurs when the
conditions are no longer so favourable to growth.
Similarly, where there is no alternation, " new in-
dividuals are usually not formed while the preceding
individuals are still rapidly growing—-that is, while
the forces producing growth exceed the opposing
forces to a great extent; but the formation of new
individuals begins when nutrition is nearly equalled
by expenditure/'

In illustration Spencer points to facts like the follow-
ing : " Uniaxial plants begin to produce their lateral,
flowering axes, only after the main axis has developed
the great mass of its leaves, and is showing its
diminished nutrition by smaller leaves, or shorter in-
ternodes, or both "; " root-pruning " and " ringing,"
which diminish the nutritive supply, promote the
formation of flower-shoots; high nutrition in plants
prevents or arrests flowering.

Similarly, the aphides or green-flies, hatched from
eggs in the spring, multiply by parthenogenesis
throughout the summer; with extraordinary rapidity
one generation follows on another; but when the
weather becomes cold and plants no longer afford
abundant sap, males reappear and sexual reproduction
sets in. It has been shown that in the artificial
summer of a green-house, parthenogenesis may con-
tinue for four years. In a large number of cases of
ordinary reproduction, e.g. in birds, the connexion
between cessation of growth and commencement of
reproduction is very distinct.

It is not difficult to see the advantages in the postpone- (par-