e
question, it was almost universally discredited.[7]

[Footnote 7: Needham, writing in 1750, says:--

"Les naturalistes modernes s'accordent unaninement a etablir, comme une
verite certaine, que toute plante vient do sa semence specifique, tout
animal d'un oeuf ou de quelque chose d'analogue preexistant dans la
plante, ou dans l'animal de meme espece qui l'a produit."--_Nouvelles
Observations_, p. 169.

"Les naturalistes out generalemente cru que les animaux microscopiques
etaient engendres par des oeufs transportes dans l'air, ou deposes dans
des eaux dormantes par des insectes volans."--_Ibid._ p. 176.]

But the skill of the microscope makers of the eighteenth century soon
reached its limit. A microscope magnifying 400 diameters was a _chef
d'oeuvre_ of the opticians of that day; and, at the same time, by no
means trustworthy. But a magnifying power of 400 diameters, even when
definition reaches the exquisite perfection of our modern achromatic
lenses, hardly suffices for the mere discernment of the smallest forms of
life. A speck, only 1/25th of an inch in diameter, has, at ten inches
from the eye, the same apparent size as an object 1/10000th of an inch in
diameter, when magnified 400 times; but forms of living matter abound,
the diameter of which is not more than 1/40000th of an inch. A filtered
infusion of hay, allowed to stand for two days, will swarm with living
things among which, any which reaches the diameter of a human red blood-
corpuscle, or about 1/3200th of an inch, is a giant. It is only by
bearing these facts in mind, that we can deal fairly with the remarkable
statements and speculations put forward by Buffon and Needham in the
middle of the eighteenth century.

When a portion of any animal or vegetable body is infused in water, it
gradually softens and disintegrates; and, as it does so, the water is
found to swarm with minute active creatures, the so-called Infusorial
Animalcules, none of which can be seen, except by the aid of the
microscope; while a large proportion belong to the category of smallest
things of which I have spoken, and which must have looked like mere dots
and lines under the ordinary microscopes of the eighteenth century.

Led by various theoretical considerations which I cannot now discuss, but
which looked promising enough in the lights of their time, Buffon and
Needham doubted the applicability of Redi's hypothesis to the infusorial
animalcules, and Needham very properly