TRANSACTIONS OF SECTION I. 891 
details of the process, with which I will not trouble you, do not afford any clear 
evidence that the nerve-fibres of the vagus pick and choose amongst the nerve- 
cells of the superior cervical ganglion; the fibres appear rather to form their 
terminal branches around any kind of nerve-cell, so that, in fact, the action which 
the nerve-fibre will in future bring about depends not on any intrinsic character 
of its own, but upon the nature of the action carried on by the nerve-cell. The 
nerve-cell may cause secretion from a gland, or contraction of a blood-vessel, or 
dilation of the pupil, or movement of hairs; whichever action it causes the nerve- 
fibre which joins it from the vagus nerve can cause for the future, and it can cause 
no other. In this case, then, we arrive at results which are hopelessly at variance 
with the view that the nerve-fibres and nerve-cells of the involuntary nervous 
system are divided into classes which are fundamentally different. In other words, 
that theory which is spoken of as the theory of specific nerve energy does not 
apply here. 
But if this is so, how are we to account for the selective power shown by the 
sympathetic nerve-fibres which I have mentioned earlier? ‘That the diiferent classes 
of nerve-fibres and nerve-cells with which we are dealing have not those deep and 
inherent differences which are required by the theory of specific nerve-energy is, 
it seems to me, certain. Nevertheless, there may be some differences of a com- 
paratively superficial nature which suffice to explain the selective activity observed. 
‘We may suppose that a re-growing nerve-fibre will in favourable circumstances 
join a nerve-cell, the function of which is the same as that of its original cell, but 
that if there are hindrances in the way of this return to normal action, and if the 
conditions are favourable for joining a nerve-cell acting on some other tissue, why 
then it will join this. It is as ifit had a preference, but did not care overmuch. 
We might perhaps express the facts by saying that there are different varieties of 
pre-ganglionic fibres, but no species. 
We have been speaking so far of the nerve-fibres which run from the brain 
and spinal cord to the peripheral nerve-cells. The nerve-fibres which run from 
‘the peripheral nerve-cells have also, there is reason to believe, a large measure of 
indifference as to the kind of work they perform, The limits of this indifference 
have yet to be investigated. 
I have said earlier that in mammalia nerve-fibres are not known to run to 
connective-tissue cells or to epidermic cells. But in some lower vertebrates certain 
connective-tissue cells are under the control of the central nervous system. Thus 
in the frog the pigmented connective-tissue cells, which play a large part in deter- 
mining the colour of the skin, can be made to contract or to rearrange their pig- 
ment granules—and so change the colour of the skin—by excitation of certain 
nerves, In all probability the motor nerve-fibres to the pigment-cells belong to 
the same class as the nerve-fibres which run to the arteries and to the glands—c.e. 
they belong to the autonomic system. We have seen that unstriated muscle- 
cells and gland-cells in different parts of the body are by no means equally supplied 
with motor nerve-fibres, and it may be that in mammals there are certain con- 
nective-tissue cells which receive motor nerve-tibres. Further, if it is true, as it 
well may be, that nerve-fibres which run to a gland are capable in favourable 
conditions of making connections with a blood-vessel, it is not beyond hope that 
either kind of nerve-fibre may experimentally, by offering it favourable conditions, 
be induced to join connective-tissue cells. 
The factors which determine whether a particular tissue or part of a tissue is 
eventually supplied with nerve-endings, and the degree of development of these, 
are the factors which determine evolution in general. In the individual it is 
exercise of function which leads to the development of particular parts; in the 
race it is the utility of this development which leads to their preservation. And so 
it is conceivable that in some lower vertebrate at some time, the autonomic nervous 
system may have developed especially in connection with those tissues which appear 
in ourselves to be wholly unprovided with motor nerve-fibres. 
I am tempted, before ending, to make a slight digression. Those who have 
occasion to enter into the depths of what is oddly, if generously, called the litera- 
ture of a scientific subject, alone know the difficulty of emerging with an unsoured 
