Stimulus & Mechanism as Factors in Organisation. 22 1 
A very significant feature in regeneration lies in the relation 
between the external form of the regenerated part and its internal 
structure. In plants, serious injury is often followed by the for¬ 
mation of a tissue which, in many of its properties, resembles that 
of the embryonic regions. Out of this new organs arise, and new 
tissues are differentiated, like those of the normal parts of the plant. 
But this resemblance is not always to be met with. Lizards, for 
example, easily lose and readily regenerate, their tails, but the new 
tail though similar in general appearance to the first one is by no 
means identical with it. This is seen especially in the internal 
structure. The bony axis, for example, is very different in the two 
cases. Clearly the stimuli and mechanism concerned are not the 
same as in the embryonic animal, The regeneration is not incom¬ 
plete, it is divergent, although the general external lizard outline is 
reproduced. 
The remarkable experiments on Salamanders by which double 
hands and other monstrous developments were produced in conse¬ 
quence of appropriate lesions of the arm may also be fitly cited 
here, as they provide still other examples of the effect of stimuli on 
mechanisms whereby under the conditions of the experiments, 
useless variants on the normal may result. 
Those remarkable phenomena known as teratological are pro¬ 
bably best to be brought under consideration here, though time will 
only permit of a brief glance at their more salient features. I have 
indicated that in galls we have a clear case of the production of 
specific forms as the result of stimuli given by known agents, though 
we are as yet entirely ignorant of the actual nature of the substance 
that produces the result, or even whether it comes, as is perhaps 
probable, from the glandular excretion of the larva The mal¬ 
formations that can be induced in roses by injudicious manuring is 
another case in point. But often we are unable to track even the 
proximate agent, and it is sometimes assumed that the character must 
be of the nature of a morphological reversion. 1 confess I find 
it difficult to think out how a plant can be supposed to replace such 
a structure as an ovule by a vegetative bud, or what not, because these 
, 1 ' * 1 
are “ morphologically identical ,” for it does not replace the ovule by 
an ancestral type of shoot, but usually by one bearing the character 
of the sporting plant itself. Again, the seminiferous scale of a pine 
cone has been compared to the double needle leaves, or to a branch 
bearing these, but such dwarf-shoots are by no means characteristic 
of all conifers that exhibit double scales in their cones, as was long 
ago forcibly insisted on by Eichler. 
