812 REPORT— 1901. 



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 systenx can differ from another only in the configuration and 

 motion which it has at a given instant. To explain differences 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 muiimum 

 visible. The smallest particle of matter that can now be seen with the 

 powerfid objectives and compensating eyepieces of the present day is between 

 the jijij'trnu ^id the ^nirVnir of an inch, or 2,3^00 of a millimetre in diameter, 

 that is to say, five times smaller than the estimate of Helmholtz of j^Vrr "f 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 i^s-hxiu o^ ^ millimetre, it is possible 

 to conceive that some of the phenomena of vitality may be exhibited by a 

 body even of such small dimensions. Some of the objects now studied by the 

 bacteriologist are probably of this minute size, and it is possible that some may 

 be so small that they can never be seen. It has been observed that certain 

 fluids derived from the culture of micro-organisms may be filtered through special 

 filters, so that no particles are seen with the highest powers, and yet those fluids 

 have properties that cannot be explained by supposing that they contain toxic 

 substances in solution, but rather by the assumption that they contain a greater 

 or less number of organic particles so small as to be microscopically invisible.^ 



' The evidence upon this point is derived from pathological sources. I am 

 indebted to my friend Dr. James Ritchie, of the Pathological Institute of Oxford, 

 for the following notes : 



Notes on Organisii>s too small to he seen hy tlw Mioroscojje. 



The filters used in the work performed in the investigation of such organisms are 

 of several kinds and patterns. They arc tubes or solid cylinders made of either 

 (a) kieselguhr as in the Berkefeld filter, or (b) of unglazed porcelain as in the 

 Chamberland and Kitasato tiltcrs. They are of varying degrees of porosity accord- 

 ing to the fineness of the material used. The most porous, i.e., those which will 

 let through the largest particles, are the Berkefeld ; nest comes the Chamberland 

 ' F ' pattern ; next the Chamberland ' B ' pattern and the Kitasato tubes. All such 

 filters are used either by forcing the liquid through by pressure or by inserting 

 them into a filter flask which can be exhausted. Tlie finer kinds will keep back all 

 known bacteria. Further, as showing their mode of action, the finer kinds will not 

 allow all the constituents of such a fluid as blood serum to pass througli : a certain 

 amount of albumen is kept back. The three diseases which have been investigated 



