Stimulus & Mechanism as Factors in Organisation. 197 
albeit a complex track. When, however, it does become deranged 
we speedily observe indications of this in —an alteration of form. 
The alteration may be slight or it may be profound, and it may even 
affect the entire character of the organism. I need only mention a 
few examples to illustrate what I mean. 
Galls, as is well-known, are due to the stimulus given by the 
presence of the larvae of the gall-flies, and if these are killed the gall 
ceases to grow—the abnormal growth is certainly due partly to the 
existence of specific stimuli on the part of the animal, which not 
only can, but of necessity do, directly modify the normal sequences 
of events in the tissues affected, and so lead to growths that are 
characteristic mutually of the plant and the animal concerned. 
Different species, and in some cases different broods of the same 
species of gall flies, produce different kinds of galls, each one 
however specifically recognisable per se. The replacement of ovules 
by buds as the result of early infection by aphides is an example of 
a similar interference with normal processes by the addition of an 
abnormal stimulus of a material nature. Another remarkable 
example of form-change was discovered and elucidated by Dr. 
Blackman a couple of years ago; he found, by making Incisions into 
leaves of cherry laurel, severed from the plant, that the leaves so 
treated cut out the wounded parts at some distance from the actual 
place of injury. The character of the excision is dependent on the 
nature of the wound and the circumstances of the environment, but 
the interest, in this connection, of the discovery lies in the fact that 
the stimulus has provoked new formative activities in the living 
tissues that are situated, it may be at a considerable distance from 
the actual seat of lesion. And once more, the familiar departure 
from its terrestrial or floating form assumed by the mycelium of 
Mu cor racemosus when immersed in a nutritive medium affords 
yet another example, whilst finally I might quote the beautiful experi¬ 
ments of Klebs upon algas, in which he shewed how closely the 
appearances of the various alternative phases and forms were 
bound up with the function of nutrition (in the widest sense of the 
term), and how regularly the plants could be induced to respond to 
altered stimuli by a definite physiological change of state. 
Now although we possess at present very little precise know¬ 
ledge as to the constitution of the more complex organic substances, 
rapid strides are being made in this direction, and I believe that the 
property of form will ultimately prove susceptible of intelligible 
analysis, and that just as in the case of many carbon compounds, 
