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POPULAR SCIENCE REVIEW. 
fecundation must occur at certain fixed periods in every case of 
alternate generation : namely, when the cycle of intermediate 
“ zooids” has reached its climax; hut that in Parthenogenesis 
it may occur before every reproductive act . We eliminate, there- 
fore, from our inquiry all forms of metamorphosis and all 
kinds of reproduction (fission, gemmation, and alternate 
generation) which are not sexual. For there is no question 
of sex in the intermediate members of a series ; they are sex- 
less, because the whole broods or series of broods belong to 
one generation, and are parts of a perfect individual, of which 
the last member only of the series is the sexual representative. 
What, then, it may be asked, is meant by Partheno- 
genesis in the viviparous Aphis ? in which only imperfect (?) 
female organs have been recognized. After what has been 
said of the generation of Protozoa by conjugation and ova, it 
seems unlikely that an insect belonging to a class possessing 
so much higher a grade of organization should be sexually 
imperfect during the period of its most astonishing fertility, 
and that its mode of reproduction should fall so far back in 
standard as that of internal gemmation. For it is clear that 
the immediate progeny of the oviparous insect cannot com- 
pare in numbers with the ten or more broods of the summer 
(viviparous) insect. Nor is the Aphis a solitary exception 
amongst insects. The ant, bee, cochineal insect, cynips, silk- 
worm, several lepidoptera, and probably many others, exhibit 
the same phenomena of nonsexual reproduction ; and the same 
anomaly must, therefore, be assumed for all. Moreover, Par- 
thenogenesis has been observed in the higher invertebrates 
(crustacea) as well as in lower animals (mollusca, and lately 
by Leuckart in a nematoid worm). The doctrine which was 
invented as an explanation of the Aphis larvation has become 
in fact a basis for a widely extended scheme of reproduction. 
To the support of this doctrine two kinds of facts have been 
applied : namely, facts which establish the production of pro- 
geny by creatures under circumstances in which sexual congress 
was impossible ; and facts determined by dissection which de- 
clare for or against the assumed imperfection of female and 
absence of male organs. Besides these facts, a physiological 
question is involved ; namely, within what limits can the vital 
energy or prolific power of an unfertilised ovum be accepted ? 
Now the first class of facts is unimpeachable in the case of the 
Aphides as well as most of the other instances mentioned. 
The second class of facts is still “ sub judice. 3 3 And as to the 
physiological inquiry, it obviously waits on the determination 
of the anatomical facts. The setting up of any hypothesis of the 
kind alluded to appears of questionable advantage, as it tends 
to lead away from facts to discussions without a base. In 
