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THE BEARING OF EXPERIMENTAL EVIDENCE UPON THE GERM-THEORY OF DISEASE. SIR,-Dr. Bastian's remarks on the above subject, published in the last number of the BRITISH MEDICAL JOURNAL, will receive the attention due to any deliberate statement made by so high an authority. I question, however, if pathologists will be impressed with either the clearness or the cogency of the arguments adduced by Dr. Bastian against the germ-theory of disease; or will regard as other than rash and unwarranted his statements, " that existing evidence seems abund antly sufficient for the rejection of this doctrine as untrue", and that " all the distinctive positions of those who advocate a belief in the so called ' germ-theory of disease', or rely upon the exclusive doctrine of a ' contagium vivum', seem to be absolutely broken down and refuted". Before we can be in a position to talk so decidedly and confidently re garding " the distinctive positions of" those who maintain the germ theory of disease, we must have an intelligent appreciation of what those distinctive positions are. To understand or appreciate arguments for or against the germ-theory of disease, we must have a clear understanding what that theory is. Till we have this, we cannot see the exact force and bearing of a given argumeint. What, then, is the germ-theory of disease?

 This is no unnecessary question; for, both by its friends and foes, this theory is referred to in the most loose and indefinite manner. Not only by Dr. Bastian, but by many others, the germ-theory of fermentation, and the germ-theory of disease, are treated and written about as parts of one and the same question. It cannot be too strenuously insisted on that the two questions, though allied, are totally distinct and separate ; and that each must stand or fall on its own merits, and independently of the other. A short statement of what each theory implies will make this ap parent. There are two theories as to the mode of production of ferment ation. One is the vital theory, according to which living organisms are the sole causes capable of producing fermentative change. The other is the physico-chemical theory, according to which such change may be produced by any organic matter, living or dead, which is undergoing change. All admit the presence of organisms in the fermenting fluid. According to one theory, these organisms are developed from the pre existing germs whose presence in the fluid gave rise to the fermentative change. According to the other, they are developed de novo in the changing fluid. Such, briefly, are the two opposing views. The question is, not whether living organisms can set agoing fermentation-that is admitted by all-but whether or not living organisms are the sole causes capable of doing so. Of the invariable presence of masses of minute organisms in fermenting fluids there is no doubt. 

The point of discussion is the source and mode of origin of these organisms. Thus it has come to pass that the discussions which have taken place regarding the mode of production of fermentation (and putrefaction) have really been dis cussions on the doctrine of spontaneous generation. Those who uphold the germ-theory of fermentation deny the occurrence of heterogenesis, and maintain that all life springs directly from antecedent life. Those who uphold the physico-chemical theory as strenuously maintain the doctrine of heterogenesis. The germ-theory of fermentation is thus essentially antagonistic to the doctrine of heterogenesis; and the question with which we are really dealing, when we discuss that theory, is the doctrine of spontane ous generation. It is not so with the germ-theory of disease. This theory is that many diseases (notably epidemic and contagious diseases) result from the propagation in the system of minute organisms having no part or share in its normal economy. It deals simply with the com petence of living organisms to produce the phenomena of disease; and does not necessarily take cognisance of the question, whether or not organisms may originate de novo. 

The germ-theory of disease, there fore, exists on an independent footing ; and would still exist and hold sway, though the physico-chemical theory of fermentation and putre faction had been proved beyond a doubt. The competence of germs to produce fermentation is admitted by all. The competence of germs to produce the phenomena of disease is the special subject of discussion between the supporters and the opponents of the germ-theory of disease. I have elsewhere* shown that the germ-theory affords of the pheno * The Germ-Theory aA4lied to the Exzfanationt of the Phenomena o1 Disease London: I876. mena of the specific fevers a more v-mplete and satisfactory explanation than any theory hitherto advanced; *he whole of the phenomena pre sented by these diseases being quite explicable on the view that millions of minute organisms are being propagated in the system during the period of their continuance. And this i% the line of argument to which we must sooner or later come, if we would fulfil the highest duties of our calling. It is not enough to try to explain why infusions of meat decompose, why wine ferments, and why urine becomes putrid. We are constantly surrounded by ailments whose causation and pathology the germ theory of disease seeks to explain. People are annually dying of these ailments by tens of thousands. It is the investigation of tbe phenomena of these diseases, as they present themselves at the bedside and in the post mortemt room, that should engage the attention of physicians and pathologists. Earnest hard-working men of science are eagerly engaged in investigating the more purely scientific, and, to humanity, less prac. tically useful, question of the germ-theory of fermentation and putre. faction. Let us, as medical men, have an equally keen sense of the importance to our science of the allied question of the germ-theory of disease-a theory which deals with the question of the causation and pathology of the most important ailments to which man is liable. But let us free ourselves at once from the fatal error of supposing that our question is inseparably linked with that of the philosophers, or that we are in any way dependent on them. There are in our ranks earnest, able, and willing workers enough. Let us only have a clear conception of what we are called upon to do. Let us be alive to the great fact that the subjects of our investigations and inquiries are the phenomena of disease, as they present themselves to our notice at the bedside ; and let us discard for ever the deluding idea that the germ-theory of disease must stand or fall with the germ theory of fermentation, or that it has even any necessary connection with it. What has been said affords all the answer that is required to Dr. Bastian's hostile remarks on the germ-theory of disease. 

The argu ments which he brings forward are all meant to support his well-knowv views on the doctrines of heterogenesis and archebsosis. They have no bearing on the germ-theory of disease. Dr. Bastian refers to the ex periments of Drs. Lewis and Cunningham, and of Dr. Burdon Sander son. It would not be difficult to show that their facts tell more in favour of than against the germ-theory of disease. On a future occa sion, I may take the opportunity to do so. In the meantime, as Dr. Bastian merely adduces them " to show that, under certain conditions, low independent forms of life may originate in the midst of living tissues, previously free from them, by a kind of transformation (hetero genesis) of some of the units of protoplasm, which, though still living, have been modified in nature and tendency by reason of their existence in a partially devitalised area". As this is all that he adduces them for, I shall content myself with pointing out that he is arguing, not against the germ-theory of disease, but in support of the doctrine of heterogenesis;