s, is exceptional. We must distinguish cases in which
it regularly occurs as part of the life-history of an animal or plant
from cases in which it has been successfully brought about by
experimental "artificial" methods designed by man. The plant-lice
"naturally" reproduce through the summer by unfertilised eggs
producing only females, but in the first cold of autumn males are
hatched from some of the eggs, and the eggs of this generation are
fertilised and bide through the winter, hatching in the following
spring. Some few moths and flies also reproduce naturally during
summer by unfertilised eggs, and the brine-shrimps and some other
fresh-water shrimps produce "fatherless" broods from their eggs,
sometimes for years in succession, until "one fine day" some males are
hatched, owing to what causes we do not know. The queen bee naturally
and regularly lays a certain number of unfertilised eggs, and these
produce, not females as do the unfertilised eggs of plant-lice, etc.,
but male bees--the drones--and it is only from such eggs that the
drones of bees are born. These are the chief cases of regular and
natural parthenogenesis, but there are others which might be
enumerated.

On the other hand, examples of artificially induced development of
eggs, not fertilised, are very few. The first known came accidentally
to notice. Female silkworm moths reared in confinement sometimes lay
eggs when kept apart from the male, and these have been found to
hatch, and give rise to caterpillars, which were not reared to
maturity. Other moths bred by collectors behaved in the same way, but
the grubs were reared to maturity, and three successive generations of
"fatherless" moths were obtained. In these cases the hatching of
unfertilised eggs is not known to occur in a state of nature, although
it probably occurs occasionally. It has also been observed--an
important fact when considered with the history of the frog's egg and
the needle--that "brushing" the unfertilised eggs of the silkworm and
other moths, that is to say, gently polishing the little egg-shells
with a soft camel's-hair brush, has the effect of starting
development. Taking two lots of unfertilised eggs adhering to slips of
paper, as laid by the mother moth, it is found that those gently
brushed will hatch, whilst those not brushed will either not hatch at
all, or in very small number. The brushing seems to disturb the
equilibrium of the protoplasmic egg-cell within the egg-s