NEMATOCERA. 43. 
The explanation of this alternation of asexual and sexual genera- 
tions is the same as that given to. the phenomena observable in the 
Aphides. 
This parthenogenetic development has a very limited range in the 
animal kingdom, being found only in Arthropods and Rotifers. It 
has no doubt been derived in a secondary way from sexual repro- 
duction. Some most remarkable cases are found in insects such as 
bees and saw-flies, in which ¢’s are produced without any fertiliza- 
tion, contrary to the usual rule, where ?’s are produced when no 
fertilization has taken place, and these give rise to further generations 
of females. Eventually through a change of climatic conditions and 
food both males and females are produced. 
The existence of sexual with asexual parthenogenetic reproduction 
has led to that remarkable phenomenon in the animal kingdom 
called “alternation of generations,” so typically displayed in the 
Ccelenterates and Annelids. ‘These furnish us with the most simple 
forms of “alternation of generations,” viz., a sexual form giving rise 
to an asexual form which, by a process of budding, gives rise to 
sexual forms like the one from which it started. This form of 
alternation is éasily understood. The ancestors of the species 
- originally produced themselves sexually and by budding at the same 
time, but most likely at different seasons of the year. By degrees 
the two modes of reproduction became confined to separate indi- 
viduals, and then gradually the dissimilarity in habits caused their 
structure to become considerably modified ; and thus gradually a 
compiete alternation of generations became established. 
A closely-allied mode to the one above (sexual and budding) is. 
seen in the case of those animals where sexual reproduction alter- 
nates with parthenogenetic reproduction, or with larve that may 
produce sexually or parthenogenetically. The Aphides may be 
taken as an example of this. In this case the ova develop partheno- 
genetically in the oviduct of the female, and as long as warm weather 
and plenty of food remain this mode of reproduction continues, but 
as soon as these conditions fail, sexual ( and ?) individuals appear. 
We may assume here the capacity of the females for producing 
parthenogenetic young without the intervention of the male element 
has been found advantageous, and has been taken hold of and 
moulded by natural selection and has led to the permanent pro- 
duction of viviparous parthenogenetic young, which only change to 
male and female forms at the cessation of favourable conditions. 
The same explanation applies to the Cecidomyia larve as mentioned. 
above. - 
