d locality that we can study and
accurately determine their organization and habits,--are unquestionably
produced from parents of their own kind. Only the minute microscopic
animals are now supposed to be generated spontaneously; and this alleged
fact rests not on direct proof, but only on our inability in certain
cases to trace the process of their production in the ordinary way. As
many of these animals, in their perfect state, are not more than the
twelve thousandth part of an inch in diameter, it is not much to be
wondered at, that we should not be able in all cases to discover their
ova, or to follow these ova through all their stages of development into
the complete being. It is farther remarkable, that these animalcules,
when once produced, whether by spontaneous or natural generation, are
all found to be provided with the organs or requisite means for
continuing their species, and, in fact, for multiplying their number
from themselves with astonishing rapidity. As they certainly have
children, it seems reasonable to suppose, according to the analogy of
all the higher animated tribes, that they also had parents. The ancients
supposed, that the worms and insects which appear in decaying organic
matter were generated there by the decomposition of the substance,
without the previous agency of individuals of the same stock. Every
schoolboy is acquainted with Virgil's mode of obtaining a new swarm of
bees from the decaying carcass of a heifer. Subsequent researches, made
with more care, and perhaps with better instruments of observation, have
entirely disproved the hypothesis, and show that the maggots were
produced in every case from eggs deposited by flies or other insects,
and were afterwards themselves developed into the state of perfect
insects. Then it seems reasonable to believe, that the improved
observations of future times will clear up the only remaining
difficulty, and show how the infusory animalcules also are generated
from beings of their own kind.

These minute creatures are prolific to a degree that transcends all
calculation; and they exist, either in the egg or maturely developed, in
inconceivable numbers. A single wheel-animalcule, _Hydatina senta_,
which was watched for more than eighteen days, and which lives still
longer, is capable of a fourfold increase in twenty-four or thirty
hours; a rate of propagation which would afford in ten days a million of
beings. From their tenacity of life, extraor