Francis Schulze in 1836, by a carefully devised experiment, struck another blow at Needham’s theory of spontaneous generation. In 1837 Schwann convinced himself that the cause of decomposition must exist in the air. Schroeder and Van Dusch in 1854 proved that filtration of the air through cotton-wool was effectual in excluding germs. Then Hoffman in 1860, and Chevreuil and Pasteur working independently in 1861, showed that a sterile solution could be kept sterile if the neck of the vessel were bent in the form of an S, so that the micro-organisms in the air entering the neck of the flask, would be deposited by gravitation in the curve.

But the advocates of the theory of spontaneous generation were not yet satisfied. They objected that by the boiling of the infusions, etc., under examination they lost the ability to become decomposed; but it was shown that the admission of unfiltered air set up decomposition. Pasteur, Burdon Sanderson, and Lister next showed that blood, urine, and milk would not decompose if proper precautions were taken to avoid contamination. In 1872 Charlton Bastian endeavoured to rehabilitate the spontaneous generation theory, but Tyndall effectually disposed of his contentions. It is settled that bacteria, or microbes, as these germs are now called, when once destroyed by heat and by certain chemical agents in any medium, cannot be resuscitated, and that Harvey’s axiom, omne vivum ex ovo, applies to all forms of organisms. As Dr. Sims Woodhead has said[1046] concerning the battle between the advocates and opponents of the spontaneous generation theory:—

“The triumphs of surgery, of preventive inoculation of hygiene in relation to specific infective diseases, of preservation of food, have had their origin in the knowledge gained during the battle which waged round the question of spontaneous generation or generatio æquivoca; and to the disciples of that school every acknowledgment must be made and due credit assigned for the attitude of scepticism, and free, ingenious, and honest criticism which they passed concerning half-formed and inadequately-supported theories and imperfectly-conducted experiments, for to their efforts is certainly due the fact that the experiments of their opponents became more and more perfect, and if to-day we have perfect methods of sterilization and of making pure cultivations, it is because nothing was taken for granted, and because able men on both sides of the controversy were ranged against one another to fight the matter to the death.”

Another question which had to be determined was whether these organisms were of the animal or vegetable kingdom. Ehrenberg came to the conclusion that in consequence of snake-like and rotary movements of certain micro-organisms they were animals; and this opinion held its ground till Davaine decided that bacteria must be considered as belonging to the vegetable kingdom. Up to 1852 the animal theory was unshaken; in 1854 Cohn demonstrated the plant nature of bacteria.

In 1857 Naegeli made a group of all the forms of lesser minute organisms, and termed it Schizomycetes, or fission fungi. The connection between micro-organisms and disease was the subject of research also in another direction. The discovery by Latum and Schwann in 1837, that the yeast plant is a living organism, and the true cause of fermentation, threw great light on the whole inquiry. Many observers had long recognised the likeness of certain diseases to fermentation processes, and it gradually became the opinion that such diseases were similarly produced. In 1837 Bassi discovered that the silk-worm disease was due to microscopic spores on the bodies of sick worms, and that healthy worms became diseased when these spores were conveyed to them. Henle in 1840 declared that all contagious diseases must be caused by the growth of something of a living nature, although he had searched in vain for the living contagion of small-pox and scarlet fever. When fungi were found to be the cause of favus, herpes tonsurans, and pityriasis versicolor, the theory received a still greater impetus. Swaine, Brittan, and Budd found micro-organisms in connection with cholera. In 1857 Pasteur demonstrated that lactic, acetic, and butyric fermentations were produced by micro-organisms.[1047] In 1863 Davaine came to the conclusion that the disease known as splenic fever is caused by an organised being which kills the animal by multiplying in its blood, and so changing its nature, after the manner of a fermentation process. Pasteur next took up the investigation of silk-worm disease, and was ultimately able to confirm the opinion that the disease was due to micro-organisms, and to devise a remedy for it.

Robert Koch in 1877 described the life-history of the bacillus of anthrax or splenic fever. Pasteur also devoted much attention to the same subject, and confirmed the observations of Koch. Paul Bert, on the other hand, argued that the bacilli were of no importance. Ultimately he was convinced of his error by Pasteur; it was, however, says Professor Cruikshank,[1048] “principally the researches of Koch which placed the doctrine of contagium vivum on a scientific basis. Koch elevated the theory of contagium vivum to a demonstrated and established fact.”

The whole matter is beset with fallacies. Because certain bacteria have been discovered in the blood of animals suffering from a particular disease, it must not be rashly concluded that these bacteria are always its cause, they may be in some cases only its effects. At the present time the nature of the contagion in many diseases, such as hydrophobia, variola, vaccinia, scarlet fever, and measles, has not been discovered. The comma-bacillus is associated with cholera in some mysterious manner, yet experimenters have swallowed myriads of comma-bacilli, and have remained never the worse. Although Pasteur’s prophylactic treatment against hydrophobia is based upon the theory that a micro-organism is the cause of the disease, Pasteur has never yet discovered the bacterium of hydrophobia, yet there would seem to be one. Dr. Sims Woodhead says:[1049] “It is a most remarkable fact that although no micro-organisms can be found in the virus, filtration through the Pasteur filter keeps back the effective part of the virus, whilst heating to 100°C. destroys the activity of the virus.”

The disease-demon has now reappeared in the form of a germ.

The Phagocyte Theory.

Some thirty-six diseases, many of which are amongst the most terrible which afflict men and animals, are attributed by bacteriologists to micro-organisms.[1050] It is sufficiently alarming to reflect that enemies which can only be detected by a specialist armed with a powerful microscope are everywhere around us, waiting to attack us in a favourable spot, and slay us without hope of escape.