608 . KEPORT — 1889. 



anatomical difference between cerebro-spinal nerves of different functions. We 

 may therefore anticipate that the future of physiolog'y will differ from the past 

 chiefly in this respect — that whereas hitherto the greater part of the work has 

 consisted in the interpretation of facts arrived at in the first instance by anatomical 

 methods of research, Histolog-y, once the guide of Physiology, has now become her 

 handmaid. 



During the last ten or fifteenyearshistology has carried her methods of research 

 to such a degree of perfection that further improvement scarcely seems possible. 

 As compared with these subtle refinements, the ' minute anatomy ' of thirty years 

 ago appears coarse — the skill for which we once took credit seems but clumsiness. 

 Notwithstanding, the problems of the future from their very nature lie as com- 

 pletely out of reach of the one as of the other. It is by diflerent methods of 

 investigation that our better equipped successors must gain insight of those vital 

 processes of which even the ultimate results of microscopical analysis will ever be, 

 as they are now, only the outward and visible signs. 



The Invisible Mechanism of Life. 



In what has preceded I have endeavoured to show that at present the funda- 

 ■ mental questions in physiology, the problems which most urgently demand 

 solution, are those which relate to the endowments of apparently structureless 

 living matter, and that the most important part of the work of the immediate 

 future will be the analysis of these endowments. \Vith this view what we have 

 to do is, first, to select those cases in which the vital process offers itself in its 

 simplest form, and is consequently best understood; and, secondly, to inquire how 

 far in these particular instances we may, taking as our guide the principle I 

 have so often mentioned as fundamental, viz. the correlation of structure with 

 function, of mechanism with action, proceed in drawing inferences as to the mecha- 

 nism by which these vital processes are in these simplest cases actually carried 

 out. 



The most distinctive peculiarity of living matter as compared with non-living 

 is that it is ever changing while ever the same, i.e. that life is a state of ceaseless 

 change. For our present purpose I must ask you, first, to distinguish between two 

 kinds of change which are equally characteristic of living organisms, namely, those 

 of growth and decay on the one hand, and those of nutrition on the other. Growth 

 the biologist calls evolution. Growth means the unfolding, i.e. development of the 

 latent potentialities of form and structure which exist in the germ, and which it 

 has derived by inheritance. A growing organism is not the same to-day as it was 

 yesterday, and consequently not quite the same now as it was a minute ago, and 

 never again will be. This kind of change I am going to ask you to exclude from 

 consideration altogether at this moment, for in truth it does not belong to Physio- 

 logy but rather to Morphology, and to limit your attention to the other kind which 

 includes all other vital phenomena. I designated it just now as nutrition, but this 

 word expresses my meaning very inadequately. The term which has been used for 

 half a century to designate the sum or complex of the non-developmental activities 

 of an organism is ' exchange of material,' for which Professor Foster has given the 

 very acceptable substitute Metabolism. Metabolism is only another word for 

 ' change,' but in using it we understand it to mean that, although an organism in 

 respect of its development may never be what it has been, the phases of alternate 

 activity and repose which mark the flow of its life-stream are recurrent. Life is 

 a Cyclosis in which the organism returns after every cycle to the same point of 

 departure, ever changing yet ever the same. 



It is this antithesis which constitutes the essential distinction between the two 

 great branches of biolog}', the two opposite aspects in which the world of life 

 presents itself to the inquiring mind of man. Seen from the morphological side 

 the whole plant and animal kingdom constitutes the unfolding of a structural plaa 

 which was once latent in a form of living material of great apparent simplicity. 

 From the physiological side this apparently simple material is seen to be capable 



