Stimulus & Mechanism as Factors in Organisation. 195 
detailed apprehension. Moreover a considerable body of facts are 
not easily reconciled with it. For if polarity really means anything 
it ought to be a definite and not a variable property; hut the 
reversal of irritable properties ( as for instance when the positive 
heliotropic character of a structure becomes changed into a negative 
response, or when the apex of a root turns into that of a shoot, as 
Goebel shewed might happen in Anthurium) seems opposed to 
explanations based on polarity as a working hypothesis. But 
although the analogies with simpler phenomena are difficult to accept 
as even approximate explanations of the facts; still if we cast a 
glance over the essential and more striking characteristics of living 
bodies, our attention is immediately arrested by certain salient and 
significant features. There is the property of specific form, and the 
equally remarkable one of the general occurence of symmetry, on 
which insistence has so often been laid in the past. But constancy 
of form would seem to imply that the constituent particles are held 
together by the definite and continuous operation of systems of 
forces the complexity of which may depend on the nature of the 
substance or substances involved, and the conclusion is enormously 
strengthened by considerations drawn from a study of symmetry 
and metamerism. One can hardly doubt that the units or 
metameres form closed systems, and that their form is, at least in 
part, the expression of the kind of matter of which they are made up. 
In the inorganic world we know of simple substances that are 
capable—like sulphur—of exhibiting several crystalline forms, each 
depending perhaps on a different molecular arrangement, and we 
know too of many cases in which an infinitesimally small admixture 
of a foreign substance may effect the properties of the whole in 
important respects. In the world of living things it is not then to 
be wondered at that organisation is no simple matter. I need only 
refer to the deviations round a mean form, and the slight distortions 
of it, which at first sight seem so sharply to mark off organic shape 
from the definiteness characteristic of crystals. But we know of 
pseudo-crystals of organic substances in which there is not that 
degree of mathematical constancy that characterises such a sub¬ 
stance as, e.g. common salt. Possibly the variability here depends 
on local variations in composition. But though the lability, semi¬ 
fluidity and other characters of an organism, perhaps, seem to 
forbid a comparison with crystalline form, I do not think we are 
unwarranted in seeking to find one. 
The forms of crystals owe their origin and their properties (as 
