As our present physicians are divided on these two systems of spermatic worms and eggs, and as all those who have lately written on generation have adopted one or the other of these opinions, it seems necessary to examine them with care, and to shew that they are not only sufficient to explain the phenomena of generation, but are also founded on suppositions void of all probability. Both suppose an infinite progression; which, as we have said, is not so much a reasonable supposition as an illusion of the mind. A spermatic worm is more than a thousand million times smaller than a man; if, therefore, we suppose the body of a man as an unit, the size of the spermatic worm can only be expressed by the fraction 1/1000000000; and as man is with respect to the spermatic worm of the first generation, what this worm is to that of the second generation, the size of the last spermatic worm cannot be expressed but by a number composed of nineteen cyphers; and so likewise the size of the spermatic worm of the third generation will require 28 cyphers; that of the fourth generation 37; the fifth 46, and the sixth 55 cyphers. To form an idea of the minuteness represented by this fraction, let us take the dimensions of the sphere of the universe from Sol to Saturn, and supposing the sun a million times larger than the earth, and about a thousand solar diameters distant from Saturn, we shall perceive that only 45 cyphers are required to express the number of cubic lines contained in this sphere; and, by reducing each cubic line into a thousand millions of atoms, 54 cyphers are only required to express that number; consequently a human being will be greater, with relation to a spermatic worm of the sixth generation, than the sphere of the universe is with relation to the smallest atom which is possible to be perceived by the assistance of a microscope. What would it be if we were to carry it to ten generations? The minuteness would be so great as to leave us no mode of expressing it. The probability of this opinion, therefore, evidently disappears in proportion as the object diminishes. This calculation may be applied to eggs as well as spermatic worms, and the want of probability is general to both; it will, no doubt, be said, that matter being divisible, _ad infinitum_, there is no impossibility in this diminution of size; and although it is not probable, yet we must regard this division of matter as possible, since we can always, by thought, divide an atom into a number of parts. But I answer, that the same illusion is made use of on this infinite divisibility as on every other geometrical and arithmetical infinity; they are only abstractions of the mind, and have no existence in nature. If we look on infinite divisibility of matter as an absolute infinity, it is easy to demonstrate that in that sense it does not exist; for, if once we suppose the smallest atom possible, by that supposition this atom will