546 



NATURE 



[September 26, 1901 



locomotion, the circulation of the blood, respiration, digestion, 

 the mechanism of the senses, and the general phenomena of the 

 nervous system have all been investigated, and in a general way 

 they are understood. The same statement may be made as to 

 the majority of individual organs. It is when we come to the 

 phenomena in the living tissues that we find ourselves in diffi- 

 culties. The changes happening in any living cell, let it be a 

 C(5nnective tissue corpuscle, or a secreting cell, or a nerve-cell, 

 are still imperfectly understood ; and yet it is upon these 

 changes that the phenomena of life depend. This has led the 

 more thoughtful physiologists in recent years back again to the 

 study of the cell and of the simple tissues that are formed from 

 cells. Further, it is now recognised that if we are to give an 

 adequate explanation of the phenomena of life, we should study 

 these, not in the body of one of the lower organisms, as was at 

 one time the fashion, where there is little if any differentiation of 

 function — the whole body of an amceboid organism showing 

 capacities for locomotion, respiration, digestion, &c. — but in the 

 specialised tissue of one of the higher animals. Thus the 

 muscle-cell is specialised for contraction, and varieties of 

 epithelium have highly specialised functions. 



But when cells are examined with the highest microscopic 

 powers, and with the aid of the highly elaborated methods of 

 modern histology, we do not seem to have advanced very far 

 towards an explanation of the ultimate phenomena. There is 

 the same feeling in the mind of the physiologist when he attacks 

 the cell from the chemical side. By using large numbers of 

 cellular elements, or by the more modern and fruitful methods 

 of micro-chemistry, he resolves the cell-substance into proteids, 

 carbohydrates, fats, saline matter and water, with possibly other 

 substances derived from the chemical changes happening in the 

 cell while it was alive ; but he obtains little information as to 

 how these proximate constituents, as they are called, are built 

 up into the living substance of the cell. But if we consider 

 the matter it will be evident that the phenomena of life depend 

 on changes occurring in the interactions of particles of matter 

 far loo small even to be seen by the microscope. The physicist 

 and the chemist have not been content with the investigation 

 of large masses of dead matter, but to explain many phenomena 

 they have had recourse to the conceptions of molecules and atoms 

 and of the dynamical laws that regulate their movements. Thus 

 the conception of a gas as consisting of molecules having a to- 

 and-fro motion, first advanced by Kronig in 1S56 and by 

 Clausius in 1S57, has enabled physicists to explain in a satis- 

 factory manner the general phenomena of gases, such as pres- 

 sure, viscosity, diffusion, Sic. In physiology few attempts have 

 been made in this direction, probably because it was felt that 

 data had. not been collected in sufficient numbers and with 

 sufficient accuracy to warrant any hypothesis of the molecular 

 structure of living matter, and physiologists have been content 

 with the microscopic and chemical examination of cells, of 

 protoplasm, and of the simpler tissues formed from cells. An 

 exception to this general remark is the well-known hypothesis 

 of Du Bois-Reymond as to the existence in muscle of molecules 

 having certain electrical properties, by which he endeavoured to 

 explain the more obvious electrical phenomena of muscle and 

 nerve. The conception of gemmules by Darwin and of biophors 

 by Weismann are examples also of a hypothetical method of 

 discussing certain vital phenomena. 



The conception, however, of the existence in living matter of 

 molecules has not escaped some astute physicists. The subject 

 is discussed with his usual suggestiveness by Clerk Maxwell in the 

 article Atom in the " Encyclopedia Britannica" in the volume 

 published in 1S75, and he places before the physiologist a curious 

 dilemma. After referring to estimates of the diameter of a 

 molecule made by Loschmidt in 1865, by Stoney in 1S6S, and 

 by Lord Kelvin (then Sir W. Thomson) in 1S70, Clerk Maxwell 

 writes : — 



" The diameter and the mass of a molecule, as estimated by 

 these methods, are, of course, very small, but by no means 

 infinitely so. About two millions of molecules of hydrogen in a 

 row would occupy a millimetre, and about two hundred million 

 million million of them would weigh a milligramme. These 

 numbers must be considered as exceedingly rough guesses ; 

 they must be corrected by more extensive ancWaccurate experi- 

 ments as science advances ; but the main result, which appears 

 to be well established, is that the determination of the mass of 

 a molecule is a legitimate object of scientific research, and that 

 this mass is by no means immeasurably small. . 



"Loschmidt illustrates these molecular measurements by a 



NO. 1665, VOL. 64] 



comparison with the smallest magnitudes visible by means of a 

 microscope. Nobert, he tells us, can draw 4000 lines in the 

 breadth of a millimetre. The intervals between these lines can 

 be observed with a good microscope. A cube, whose side is 

 the 4000th of a millimetre, may be taken as the minimum 

 visible for observers of the present day. Such a cube would 

 contain from 60 to 100 million molecules of oxygen or of 

 nitrogen ; but since the molecules of organised substances 

 contain on an average about fifty of the more elementary atoms, 

 we may assume that the smallest organised particle visible under 

 the microscope contains about two million molecules of organic 

 matter. At least half of every living organism consists of 

 water, so that the smallest living being visible under the 

 microscope does not contain more than about a million organic 

 molecules. Some exceedingly simple organism maybe supposed 

 built up of not more than a million similar molecules. It is 

 impossible, however, to conceive so small a number sufficient 

 to form a being furnished with a whole system of specialised 

 organs. 



"Thus molecular science sets us face to face with physio- 

 logical theories. It forbids the physiologist from imagining that 

 structural details of infinitely small dimensions can furnish an 

 explanation of the infinite variety which exists in the properties 

 and functions of the most minute organisms. 



"A microscopic germ is, we know, capable of development 

 into a highly organised animal, .\nother germ, equally micro- 

 scopic, becomes when developed an animal of a totally different 

 kind. Do all the differences, infinite in number, which dis- 

 tinguish the one animal from the other arise each from some 

 dift'erence in the structure of the respective germs ? Even if we 

 admit this as possible, we shall be called upon by the advocates 

 of pangenesis to admit still greater marvels. For the micro- 

 scopic germ, according to this theory, is no mere , individual but 

 a representative body, containing members collected from every 

 rank of the long-drawn ramification of the ancestral tree, the 

 number of these members being amply sufficient not only to 

 furnish the hereditary characteristics of every organ of the body 

 and every habit of the animal from birth to death, but also to 

 afford a stock of latent gemmules to be passed on in an inactive 

 state from germ to germ, till at last the ancestral peculiarity 

 which it represents is revived in some remote descendant. 



"Some of the exponents of this theory of heredity have 

 attempted to elude the difficulty of placing a whole world of 

 wonders within a body so small and so devoid of visible structure 

 as a germ by using the phrase structureless germs. Now one 

 material system can differ from another only in the configuration 

 and motion which it has at a given instant. To explain dif- 

 ferences of function and development of a germ without 

 assuming differences of structure is, therefore, to admit that the 

 properties of a germ are not those of a purely material 

 system." 



The dilemma thus put by Clerk Maxwell is (first) that the 

 germ cannot be structureless, otherwise it could not develop 

 into a future being, with its thousands of characteristics ; or 

 (second) if it is structural it is too small to contain a sufficient 

 number of molecules to account for all the characteristics that 

 are transmitted. A third alternative might be suggested, 

 namely, that the germ is not a purely material system, an 

 alternative that is tantamount to abandoning all attempts to 

 solve the problem by the methods of science. 



It is interesting to inquire how far the argument of Clerk 

 Maxwell holds good in the light of the knowledge we now 

 possess. First, as regards the iniiiiiiuiiii visible. The smallest 

 particle of matter that can now be seen with the powerful objec- 

 tives and compensating eyepieces of the present day is between 

 the 47rrrVnirth and the ^T.^Vinrth of a inch, or ^s^JrjTr.th of a milli- 

 metre in diameter, that is to say, five times smaller than the 

 estimate of Helmholtz of ^sV^ th of a millimetre. The diffraction 

 of light in the microscope forbids the possibility of seeing still 

 smaller objects, and when we are informed by the physicists 

 that the thickness of an atom or molecule of the substances 

 investigated is not much less than a millionth of a millimetre, 

 we see how far short the limits of visibility fall of the ultimate 

 structure of matter. 



Suppose, then, we can see with the highest powers of the 

 microscope a minute particle having a diameter of .; , : , \ „ n'Ca of a 

 millimetre, it is possible to conceive that some of the phenomena 

 of vitality may be exhibited by a body even of such small 

 dimensions. The spores of some of the minute objects now 

 studied by the bacteriologist arc probably of this minute size. 



