Stimulus & Mechanism as Factors in Organisation.? 19 
simple leaves, whereas they do seem more easily intelligible on the 
material hypothesis previously outlined, and secondly those 
characters that do appear purposive, and the use of which is closely 
bound up with their form and distribution, may equally well be 
regarded as the expression of the necessary activity of internal 
factors; naturally only those species whose inner character 
expressed itself in making these “suitable” adjustments to the 
environment were able to survive. 
The self-regulatory mechanism of plants, and indeed of animals 
also, at first sight may appear to be specially characteristic of living 
matter and to be far removed from chemical or physical treatment. 
A little consideration, however, will suffice to shew that so far from 
this being the case, this very quality affords a strong support to the 
notion that structure is the outcome of the operation of causes 
associated with the material constitution of the particular organism 
concerned. 
i-" 
The co ordination so often observed in the functional arrange¬ 
ments of an organ exists as the result of the appropriate chains or 
series of stimuli that of necessity originate when one part of the 
functional machinery is put into operation. The apparent co¬ 
operative action may be, in its net result, highly purposive, but to 
place this aspect of the matter in the foreground is to obscure the 
real significance of the sequence. For example, when a green leaf 
is engaged in the work of photosynthesis, some part of the soluble 
carbohydrate may perhaps diffuse out from the cell, but a consi¬ 
derable portion is temporarily changed with insoluble starch. Now 
since it is essential for its continuous production, that the excess of 
sugar should be removed and thereby prevented from arresting 
further synthetic activity, it is clear that its utilisation to form 
starch is a “purposive adaptation,” a manifestation of the co¬ 
ordinated activity of the machinery, of the highest importance. 
But it would clearly be erroneous to say (as is often loosely done) 
that the starch is formed in order that more carbohydrate may be 
constructed. Experiment shews, at least in favorable cases, that 
sugar in sufficient amount is itself a stimulus to the plastids and 
can cause them to undergo those metabolic changes which result in 
the utilisation of the sugar, with a concurrent production of starch. 
It is inevitable that any organism that chanced to exhibit such a 
kind of mechanism would stand at an advantage, when competing 
with others from which it was lacking, and the same being true at 
any phylogenetic epoch in the history of the race, would lead to the 
