10 Proceedings of the Royal Physical Society. 



Just as with the molecules or atoms of non-living substances, so we may say 

 of living substance that it is theoretically possible to go on subdividing 

 it into smaller and smaller pieces, each of which shall still be living, but that 

 eventually a limit must be reached below which the distinctive features 

 of living substance disappear. There thus naturally arises the idea of an 

 ultimate unit of living substance such as has been employed by Nageli, 

 Darwin, Weismann, and others. It seems to me extremely doubtful whether 

 the actual existence of such units is at all a necessary assumption. I see 

 no special difficulty in believing that living substance is not built up of units. 

 But whether or not this idea of ultimate living units is a sound one, I think 

 it must be confessed that similar conceptions have done considerable harm. 

 Such harmful influence has become apparent in recent years as regards the 

 cell theory. It is also, I think, apparent in the tendency shown by many 

 writers still to regard the organism as an aggregate of organs, rather than as 

 an organism in which organs are more or less distinctly differentiated. The 

 fallacious reasoning which this involves is often seen to intrude itself into 

 discussions on Natural Selection. E.g., if a special species of animal is char- 

 acterised by marked elongation of some particular organ, it is asked : " How 

 can this have arisen by Natural Selection ? It is surely impossible to believe 

 that stray variations of very minute magnitude can have really been of any 

 marked advantage in the struggle for existence." What really leads to 

 survival is not however the possession of a favourable variation in some one 

 particular organ, but " general fitness " of the organism as a whole, which is 

 a far more complicated thing. 



In, e.g., a complex Metazoan, such as a bird or a mammal, variability affects 

 probably every bit of every tissue in the body. No bit of any tissue is 

 absolutely normal. Everything varies slightly. And the comparatively fit 

 individual, the individual which is selected in the evolutionary progress 

 towards more and more perfect adaptation, is the individual in which the 

 sum of the variations in all its varying parts is comparatively favourable. 

 Very small variations in individual organs, trivial enough in themselves, yet 

 go to build up the general fitness which plays a part in Evolution. 



While acknowledging the vast debt which Zoology owes to the method 

 which breaks up the organism into its constituent parts for the purpose 

 of investigation, and while recognising the immense stimulus to research 

 afforded by such fascinating working hypotheses as the Cell doctrine, the 

 Neuron doctrine, the doctrine of Pangens, we must ever bear in mind that 

 such doctrines are working hypotheses and nothing more, which, so long 

 as they are recognised as such, may be of great service to Science before they 

 are finally relegated to the scrap heap, but which may on the other hand 



