I^ov. 29, 1877] 



NATURE 



85 



development we observe its gradual emergence. So far, 

 inference of the existence of structure from historical 

 evidence is justifiable ; but if we were to carry this 

 inference back to the ovum itself, and say that the cha- 

 racteristic structures of nerve, of muscle, or of gland, 

 exist in the ovum at the moment after impregnation, 

 every physiologist would feel the assertion to be absurd. 



In the familiar comparison of the origin of the elephant 

 with that of the mouse, in which the perfect anatomical 

 similarity of the ova in the two species is contrasted with 

 the enormous difference of the result, we should be jus:i- 

 fied in saying that the difference of development is the 

 expression of structural difference between the primordium 

 of the one and the primordium of the other ; but inasmuch 

 as it is not possible to indicate any anatomical distinction, 

 it is understood that structural difference of another kind 

 is meant, namely, difference of molecular constitution. In 

 other words, we assume that the potential difference 

 between the one and the other is dependent on an actual 

 difference of molecular structure. Whether this is accom- 

 panied with an ana'omical difference, such as we might 

 expect to be able to see if we had more perfect instru- 

 ments, we do not know. 



From the moment that it is understood that the word 

 structure means anatomical structure, the argument used 

 by Dr. Tyndall loses its relevance. After referring to the 

 " germ limit," he says, " some of those particles " (by 

 which, I presume, is meant atmospheric particles) "de- 

 velop into globular Bacteria, some into rod-shaped 

 Bacteria, some into long flexile filaments, some into 

 impetuously moving organisms, and some into organisms 

 without motion. One particle will emerge as a Bacillus 

 anthracis, which produces deadly splenic fever ; another 

 will develop into a Bacteriutn, the spores of which are 

 not to be microscopically distinguished from th )se of the 

 former organism ; and yet these undistinguishable spores 

 are absolutely powerless to produce the disorder which 

 Bacillus anthracis never fails to produce. It is not to be 

 imagined that particles which, on development, emerge in 

 organisms so different from each other, possess no struc- 

 tural differences. But if they possess structural differences 

 they must possess the thing differentiated, viz., structure 

 itself." Throughout this passage it is evident that it is 

 not anatomical but molecular structure that is referred to. 



In the other passages relating to the subject, I venture 

 to think that Dr. Tyndall has overlooked the distinction 

 made by me between anatomical organisation and mole- 

 cular structure. When, for example, he speaks of " germ 

 structure " in the passage quoted from his Liverpool 

 Address, he evidently refers to molecular structure exclu- 

 sively, for he gives ice as his first example, and argues 

 that as ice possesses structure so do atmospheric germs — 

 a proposition which I should not have thought of ques- 

 tioning. 



The experimental evidence which we have before us 

 goes to prove that in all the known cases in which Bac- 

 teria appear to originate de iwvo — that is to say in liquids 

 which are at the moment of their origin absolutely free 

 from living Bacteria — they really originate from " par- 

 ticles great or small," which particles are therefore germs 

 in the sense in which that word is used by Prof. Tyndall. 

 To illustrate the views I myself entertain, and always have 

 entertained on this question, I need only refer to my 

 paper on the origin of Bacteria, published in 1871. The 

 experiments mide by me at that time brought to light 

 the then new fact, now become old by familiarity, that all 

 exposed aqueous liquids, even when absolutely free from 

 visible particles, and all moist surfaces, are contaminated 

 and exhibit a power of communicating their contami- 

 nation to other liquids. As regards water and aqueous 

 liquids in general, I insisted on the " particulate" nature 

 of the contaminating agent, and coined for the purpose 

 the adjective I have just employed (which has been since 

 adopted by other writers), at the same time pointing out 



that the particles in question were ultra-microscopical, 

 and consequently that their existence was matter of in- 

 ference as distinguished from direct observation. Dr. 

 Tyndall has demonstrated by the experiments to which I 

 have already alluded, that the ordinary air also contains 

 germinal particles of ultra-microscopical minuteness. Of 

 the completeness at>d conclusiveness of those experiments 

 I have only to express the admiration which I, in common 

 with all others whose studies have brought them into 

 relation with the subject, entertain. That such particles 

 exist there can be no question ; but of their size, struc- 

 tural attributes, or mode of development, we know 

 nothing. 



Prof. Tyndall, I am sure by inadvertence, has accused 

 me of assuming that there is some relation between the 

 limit of microscopical visibility and what he calls the 

 molecular limit, by which I presume to be meant the size 

 of the largest molecule. Nothing that I have said or 

 written could justify such a supposition. My contention 

 is not that the particles in question are of any size which 

 can be specified, but, on the contrary, that we are not in 

 a position to form any conclusion as to their size, except- 

 ing that they are so small as to be beyond the reach of 

 observation. Dr. Tyndall has taught us, first, that the 

 optical effects observed when a beam of light passes 

 through a particulate atmosphere are such as could only 

 be produced by light-scattering particles of extreme 

 minuteness ; and, secondly, that by subsidence these par- 

 ticles disappear, and that the contaminating property of 

 the atmosphere disappears with them. He has thus 

 approximately determined for us the upper limit of mag- 

 nitude, but leaves us uncertain as to the lower ; for we 

 have no evidence that the particles which render the 

 atmosphere opalescent to the beam of the electric lamp 

 may not be many times larger than those which render it 

 germinative. Consequently, the fact that the air may be 

 rendered sterile by subsidence, while affording the most 

 conclusive proof that germinal matter is not gaseous, 

 leaves us without information as to the size of the par- 

 ticles of which it consists. 



Of each germinal particle, whether inhabiting an 

 aqueous liquid or suspended in the atmosphere, it can 

 be asserted that under conditions which occur so fre- 

 quently that they may be spoken of as general (via., 

 moisture, a suitable temperature, and the presence of 

 dead proteid matter, otherwise called organic impurity), 

 it produces an organism. If, for the sake of clearness, 

 we call the particle a and the organism to which it gives 

 rise A, then what is known about the matter amounts to 

 no more than this, that the existence of A was preceded 

 by the existence of a. With respect to A we know, by 

 direct observation, that it is an organic structure ; but 

 inasmuch as we know absolutely nothing as to the size 

 and form of a, we cannot even state that it is transformed 

 into A, much less can we say anything as to the process 

 of transformation. 



Considering that it is admitted on all hands that there 

 exist in ordinary air particles which are potentially germs, 

 it might at first sight appear needless to inquire whether 

 or not this fact is to be regarded as carrying with it the 

 admission that they must necessarily possess the other 

 attributes of organised structure. Very little considera- 

 tion, however, is requisite in order to become convinced 

 that this question stands in relation with another of 

 fundamental importance in biology— that, namely, of the 

 molecular structure of living material.^ It is not neces- 

 sary for my present purpose to do more than to indicate 

 the nature of this relation. As regards every form of 

 living matter, it may be stated that, quite irrespectively of 

 its morphological characteristics, which, as we have seen, 



' The reader who is interested in this subject will find it discussed with 

 ereat ingenuity by Prof, f Huger, in his paper " Ueber die phvsio'.ogische 

 Verbrennung in den lebendigen Organismen," Pfiilgers Atchiv, vol. x. 

 p. 300. 



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