116 The Galls of Essex; a Contribution to a
support of the metagenesis theory mentioned by Dr. Rein-
hard 34 as more probable than the dimorphic hypothesis which
had previously been foreshadowed by Osten-Sacken35 and in
Walsh's papers in the 'Proceedings' of the Entomological
Society of Philadelphia.36 Bassett writes that "from all the
above facts I infer that all our species that are found only in
the female sex are represented in another generation by both
sexes, and that the two broods are, owing to seasonal diffe-
rences, produced from galls that are entirely distinct from
each other." This hypothesis has now been proved to be the
true gall-fly history, and in the Cynipidae we have very dis-
tinct exponents of the metagenesis theory or so-called "alter-
nation of generations," which we know is so generally the
case with the Scolecida and Hydrozoa.37 Dr. H. Adler first
actually proved the fact with Neuroterus lenticularis and Spathe-
gaster baccarum,38 and this important discovery has been
followed up with other researches made with various gall-
flies on numerous small oak-trees planted in pots ; the theory
has thus been established in 19—really 38—distinct species.
This has revolutionised the whole history of the Cynipidae.
Dr. Adler's remarkable memoir,39 detailing his observations
and with notes on the structure of both the galls and gall-
34 "Die Hypothesen uber die Fortpflanzungsweise der eingeschlechtigen
Gallwespen." 'Berliner Entomologische Zeitschrift.' Vol. ix. (1865).
35 "Ueber die Gallen und andere durch Insecten hervorgebrachte Pflan-
zen-deformationen in Nord-America." By Baron Osten-Sacken. 'Stettiner
Entomologische Zeitung,' vol. xxii., pp. 405—423 (Oct.—Dec, 1861).
36 Vol. i., pp. 47—72 (Oct., 1861); pp. 241—259 (Sept., 1862). Vol. ii.,
pp. 33—49 (April, 1863) ; pp. 443—500 (March, 1864). Vol. iv., pp.
331—380 (May, 1865).
37 [The words "cyclical propagation," introduced into England by Mr.
Meldola in his Annual Address to the Essex Field Club (Jan. 12th, 1882),
to denote the idea hitherto known by the awkward phrase "alternation of
generations," are so much more convenient, and so neatly express the
true nature of the phenomena, that they should be adopted in future by
all biologists.—Ed.]
38 ' Deutsche Entomologische Zeitschrift' xxi. 209—248 (1877).
39 "Uber den Generationswechsel der Eichen-Gallwespen." 'Zeitschrift
fur wissenschaftliche Zoologie,' vol. xxxv., pp. 151—246 ; pl. x., xi., xii,
(1880).