And looking back through the prodigious vista of the past, I find no record of the commencement of life, and therefore I am devoid of any means of forming a definite conclusion as to the conditions of its appearance. Belief, in the scientific sense of the word, is a serious matter, and needs strong foundations.
To say, therefore, in the admitted absence of evidence, that I have any belief as to the mode in which the existing forms of life have originated, would be using words in a wrong sense. But expectation is permissible where belief is not; and if it were given me to look beyond the abyss of geologically recorded time to the still more remote period when the earth was passing through physical and chemical conditions, which it can no more see again than a man can recall his infancy, I should expect to be a witness of the evolution of living protoplasm from not living matter.
I should expect to see it appear under [] forms of great simplicity, endowed, like existing fungi, with the power of determining the formation of new protoplasm from such matters as ammonium carbonates, oxalates and tartrates, alkaline and earthy phosphates, and water, without the aid of light.
That is the expectation to which analogical reasoning leads me; but I beg you once more to recollect that I have no right to call my opinion anything but an act of philosophical faith. So much for the history of the progress of Redi's great doctrine of Biogenesis, which appears to me, with the limitations I have expressed, to be victorious along the whole line at the present day.
The tapeworms, [] bladderworms, and flukes continued to be a stronghold of the advocates of Xenogenesis for a much longer period. A plant may throw off bulbs, but these, sooner or later, give rise to seeds or spores, which develop into the original form.
But if we turn to pathology, it offers us some remarkable approximations to true Xenogenesis. Again, it is a matter of familiar experience to everybody that mere pressure on the skin will give rise to a corn. Now the gall, the tumour, [] and the corn are parts of the living body, which have become, to a certain degree, independent and distinct organisms. Under the influence of certain external conditions, elements of the body, which should have developed in due subordination to its general plan, set up for themselves and apply the nourishment which they receive to their own purposes.
From such innocent productions as corns and warts, there are all gradations to the serious tumours which, by their mere size and the mechanical obstruction they cause, destroy the organism out of which they are developed; while, finally, in those terrible structures known as cancers, the abnormal growth has acquired powers of reproduction and multiplication, and is only morphologically distinguishable from the parasitic worm, the life of which is neither more nor less closely bound up with that of the infested organism.
If there were a kind of diseased structure, the histological elements of which were capable of maintaining a separate and independent existence out of the body, it seems to me that the shadowy boundary between morbid growth and Xenogenesis would be effaced.
And I am inclined to think that the progress of discovery has almost brought us to this point already. I have been favoured by Mr. Simon with an early copy of the last published of the valuable "Reports on the [] Public Health," which, in his capacity of their medical officer, he annually presents to the Lords of the Privy Council. The appendix to this report contains an introductory essay "On the Intimate Pathology of Contagion," by Dr.
Burdon- Sanderson, which is one of the clearest, most comprehensive, and well-reasoned discussions of a great question which has come under my notice for a long time. I refer you to it for details and for the authorities for the statements I am about to make. You are familiar with what happens in vaccination. A minute cut is made in the skin, and an infinitesimal quantity of vaccine matter is inserted into the wound. Within a certain time a vesicle appears in the place of the wound, and the fluid which distends this vesicle is vaccine matter, in quantity a hundred or a thousandfold that which was originally inserted.
Now what has taken place in the course of this operation? Has the vaccine matter, by its irritative property, produced a mere blister, the fluid of which has the same irritative property? Or does the vaccine matter contain living particles, which have grown and multiplied where they have been planted? The observations of M. Chauveau, extended and confirmed by Dr. Sanderson himself, appear to leave no doubt upon this head. Similar experiments have proved that two of the most destructive of epizootic diseases, sheep-pox and glanders, are also dependent for their existence and their propagation upon extremely small living solid particles, to which the title of microzymes is applied.
An animal suffering under either of these terrible diseases is a source of infection and contagion to others, for precisely the same reason as a tub of fermenting beer is capable of propagating its fermentation by "infection," or "contagion," to fresh wort.
In both cases it is the solid living particles which are efficient; the liquid in which they float, and at the expense of which they live, being altogether passive. Now arises the question, are these microzymes the results of Homogenesis, or of Xenogenesis? Are they parasites in the zoological sense, or are they merely what Virchow has called "heterologous growths"?
It is obvious that this question has the most profound importance, [] whether we look at it from a practical or from a theoretical point of view. A parasite may be stamped out by destroying its germs, but a pathological product can only be annihilated by removing the conditions which give rise to it. It appears to me that this great problem will have to be solved for each zymotic disease separately, for analogy cuts two ways.
I have dwelt upon the analogy of pathological modification, which is in favour of the xenogenetic origin of microzymes; but I must now speak of the equally strong analogies in favour of the origin of such pestiferous particles by the ordinary process of the generation of like from like.
It is, at present, a well-established fact that certain diseases, both of plants and of animals, which have all the characters of contagious and infectious epidemics, are caused by minute organisms.
The smut of wheat is a well-known instance of such a disease, and it cannot be doubted that the grape-disease and the potato-disease fall under the same category. Among animals, insects are wonderfully liable to the ravages of contagious and infectious diseases caused by microscopic Fungi. In autumn, it is not uncommon to see flies motionless upon a window-pane, with a sort of magic circle, in white, drawn round them. These spore-forming filaments are connected with others which fill the interior of the fly's body like so much fine wool, having eaten away and destroyed the creature's viscera.
This is the full-grown condition of the Empusa. If traced back to its earliest stages, in flies which are still active, and to all appearance healthy, it is found to exist in the form of minute corpuscles which float in the blood of the fly. These multiply and lengthen into filaments, at the expense of the fly's substance; and when they have at last killed the patient, they grow out of its body and give off spores. Healthy flies shut up with diseased ones catch this mortal disease, and perish like the others.
A most competent observer, M. Cohn, who studied the development of the Empusa very carefully, was utterly unable to discover in what manner the smallest germs of the Empusa got into the fly.
The spores could not be made to give rise to such germs by cultivation; nor were such germs discoverable in the air, or in the food of the fly. It looked exceedingly like a case of Abiogenesis, or, at any rate, of Xenogenesis; and it is only quite recently that the real course of events has been made out. It has been ascertained, that when one of the spores falls upon the body of a fly, it begins to germinate, and sends out a process which bores its way through the fly's skin; this, having reached the interior cavities of its body, gives off the minute floating corpuscles which are the earliest stage of the Empusa.
The disease is "contagious," because a healthy fly coming in contact with a diseased one, from which the spore-bearing filaments protrude, is pretty sure to carry off a spore or two. It is "infectious" because the spores become scattered about all sorts of matter in the neighbourhood of the slain flies.
The silkworm has long been known to be subject to a very fatal and infectious disease called the Muscardine. Audouin transmitted it by inoculation. This disease is entirely due to the development of a fungus, Botrytis Bassiana, in the body of the caterpillar; and its contagiousness and infectiousness are accounted for in the same way as those of the fly-disease.
But, of late years, a still more serious epizootic has appeared among the silkworms; and I may mention a few facts which will give you some conception of the gravity of the injury which it has inflicted on France alone. According to the endosymbiotic theory, mitochondria and chloroplasts are each derived from the uptake of bacteria. These bacteria established a symbiotic relationship with their host cell that eventually led to the bacteria evolving into mitochondria and chloroplasts.
Prior to the discovery of microbes during the seventeenth century, other theories circulated about the origins of disease.
For example, the ancient Greeks proposed the miasma theory , which held that disease originated from particles emanating from decomposing matter, such as that in sewage or cesspits. Such particles infected humans in close proximity to the rotting material. In , Italian physician Girolamo Fracastoro proposed, in his essay De Contagione et Contagiosis Morbis , that seed-like spores may be transferred between individuals through direct contact, exposure to contaminated clothing, or through the air.
We now recognize Fracastoro as an early proponent of the germ theory of disease , which states that diseases may result from microbial infection. Figure 4. Ignaz Semmelweis — was a proponent of the importance of handwashing to prevent transfer of disease between patients by physicians. Semmelweis observed medical students performing autopsies and then subsequently carrying out vaginal examinations on living patients without washing their hands in between.
He suspected that the students carried disease from the autopsies to the patients they examined. His suspicions were supported by the untimely death of a friend, a physician who contracted a fatal wound infection after a postmortem examination of a woman who had died of a puerperal infection.
Although Semmelweis did not know the true cause of puerperal fever, he proposed that physicians were somehow transferring the causative agent to their patients.
He suggested that the number of puerperal fever cases could be reduced if physicians and medical students simply washed their hands with chlorinated lime water before and after examining every patient.
This demonstrated that handwashing was a very effective method for preventing disease transmission. Around the same time Semmelweis was promoting handwashing, in , British physician John Snow conducted studies to track the source of cholera outbreaks in London.
By tracing the outbreaks to two specific water sources, both of which were contaminated by sewage, Snow ultimately demonstrated that cholera bacteria were transmitted via drinking water. The work of both Semmelweis and Snow clearly refuted the prevailing miasma theory of the day, showing that disease is not only transmitted through the air but also through contaminated items.
Although the work of Semmelweis and Snow successfully showed the role of sanitation in preventing infectious disease, the cause of disease was not fully understood. The subsequent work of Louis Pasteur , Robert Koch , and Joseph Lister would further substantiate the germ theory of disease. While studying the causes of beer and wine spoilage in , Pasteur discovered properties of fermentation by microorganisms.
He had demonstrated with his swan-neck flask experiments see Figure 3 in Spontaneous Generation that airborne microbes, not spontaneous generation, were the cause of food spoilage, and he suggested that if microbes were responsible for food spoilage and fermentation, they could also be responsible for causing infection. This was the foundation for the germ theory of disease. Meanwhile, British surgeon Joseph Lister Figure 5a was trying to determine the causes of postsurgical infections.
His extremely successful efforts to reduce postsurgical infection caused his techniques to become a standard medical practice. Using these postulates, Koch and his colleagues were able to definitively identify the causative pathogens of specific diseases, including anthrax, tuberculosis, and cholera. Figure 5. Both scientists contributed significantly to the acceptance of the germ theory of disease. After suffering a fever, congestion, cough, and increasing aches and pains for several days, Anika suspects that she has a case of the flu.
She decides to visit the health center at her university. The PA tells Anika that her symptoms could be due to a range of diseases, such as influenza, bronchitis, pneumonia, or tuberculosis. Explanation: The idea of biogenesis was commonly believed before the experiments of Louis Pasteur. Jan 31, Related questions How do fossils help biologists understand the history of life on earth? Question 01b2e. On what continent did human life begin?
What percentage of all plant and animal life on planet earth is extinct? What is the correct sequence of these events, from earliest to most recent, in the evolution of Modern debates between science and religion revolve around evolution. Careful enough scrutiny has, in every case up to the present day, discovered life as antecedent to life. Dead matter cannot become living without coming under the influence of matter previously alive. This seems to me as sure a teaching of science as the law of gravitation.
Huxley, and I am ready to adopt as an article of scientific faith, true through all space and through all time, that life proceeds from life, and from nothing but life. This I do not for a moment suppose was in any way the meaning of Sir W. Thomson, who unintentionally has made it appear that Prof. Huxley comes to the same conclusion from the consideration of certain facts, as he does. So far from this re-assuring concord having an existence, I doubt if any single biologist of name of whatever philosophic tendencies would venture to assert that it is as sure a teaching of science as the law of gravitation that dead matter cannot become living without coming under the influence of matter previously alive, and conclusions derived from a consideration of a vast series of facts prohibit an evolutionist from accepting such a doctrine without the most complete and widely-reaching evidence in its favour.
And is it not the case that whilst human observation of bodies in relation to the law of gravitation is of the most vast character—embracing not only all varieties of terrestrial matter, but innumerable extra-terrestrial bodies—the human observation of the way in which living matter originates or grows, is a mere trifle so insignificant in extent that it is as a drop in the ocean? The merest fragment, as Prof.
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