the Gall-flies of the Oak. 287 
and females are in equal numbers, and that nevertheless 
parthenogenesis exists. M. Adler has observed the mode of 
reproduction of N. Vallisnierz7, and discovered in this species 
the existence of two broods a year which are entirely parthe- 
nogenetic; so that parthenogenesis, which is only excep- 
tional in the first species of this genus, has become constant 
in the second. 
Pieromalus puparum, the larva of which lives as a parasite 
in the chrysalides of various species of diurnal Lepidoptera, 
also exhibits phenomena of parthenogenesis; but the conse- 
quences of this mode of reproduction are the inverse of those 
which are observed in Nematus Vallisnier. Thus the 
virgin females chiefly give birth to males, a fact which brings 
these insects near to the bees as regards their reproduction. 
Of four chrysalides infested with larve of this Pteromalus 
produced by parthenogenesis, the first yielded 124 males, the 
second 62 males, the third 75 males and 5 females, the fourth 
45 males and 4 females. 
All these facts tend to prove that parthenogenesis among 
the Hymenoptera originates from sexual generation. It is, 
apart from that, difficult to establish any general law, because 
the results relative to the sex of the progeny are too change- 
able. Sometimes the virgin females give birth principally or 
exclusively to females, sometimes to males and females in 
apparently equal numbers, and, lastly, sometimes principally 
or exclusively to males’ In the case where the male sex 
seems to have entirely disappeared in consequence of prolonged 
parthenogenesis, there still reappears from time to time a male 
among a great number of individuals. 
M. Adler seeks to explain the origin of the alternation of 
the two different annual generations among the Cynipide. 
He assumes that at first there was probably only one genera- 
tion a year, and subsequently two identical generations. The 
modifications produced later on in these two generations are 
to be attributed to the influence of external vital conditions. 
Among the first must be placed changes of climate; for we 
know, chiefly from the observations and experimental re- 
searches of M. Weismann, that the influences of different 
climates are able to give the first impulse to the changes 
which lead to the separation of two generations. As for 
the degree of the modifications, it arises from a factor whose im- 
portance we cannot well appreciate; the special organization of 
a species presents sometimes a great disposition to vary, some- 
times a tendency to preserve its characters; so that there is 
sometimes scarcely any difference between the two alternate 
generations of the Cynipide in spite of the most varied ex- 
