OF EPPING FOREST.
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forming the means of exit for some small parasite may often be
mistaken for that of the gall-maker.
Parthenogenesis and Alternation of Generations.
Alternation of generations is an exceedingly interesting
feature occurring in the life history of those Cynipidae forming
galls on the oak. There are two generations a year, the first,
consisting of females only, is known as the Agamic generation,
the second, which includes both males and females, as the Sexual
generation.
The two generations give rise to galls which may differ
entirely both in shape, size and situation.
Those galls which mature in the spring give, rise to insects
comprising females only, which proceed to lay fertile eggs with-
out having any connection with a male. This is known as
Parthenogenesis or reproduction without male agency, the
generation of flies being Agamic. After these have oviposited
in the spring, galls arise which mature in the summer, and the
insects emerging from them comprise both males and females
(sexual generation); the latter, after copulation, lay eggs which
in turn eventually cause galls to form which are exactly similar
to those from which the agamic generation emerged in the
spring. The reason for this alternation of generations is not
known, and it would take too long to discuss the various theories
on the subject, but it seems to be intimately connected with the
extrusion of the polar bodies during the germination of the egg,
previous to the formation of the embryo. In the eggs of the
sexual generation two polar bodies are extended, while in the eggs
laid by the agamic generation only one polar body is extruded.
In all lower forms of life we find the occurrence of partheno-
genesis more common than in the higher forms. As regards the
Cynipidae there are a few species forming galls on the oak
which have only one generation a year, this generation being
agamic. It is probable that these and other agamic generations
at one time possessed males, as we find cases among the
Cynipidae forming galls on other plants where a few males are
still present.
For instance, in the case of Rhodites rosae, which forms the
common "Bedeguar" or Pincushion-gall on the rose, the males
are almost extinct, though a few are still found; consequently