I think Charles Darwin must not have been acquainted with this explanation of trembling, as I am convinced he would otherwise have tried to combat it, or would at least have mentioned it in his writings. He was much too conscientious to ignore an objection made to his theories.
Here we have the example of another phenomenon which seems to contradict certain hypotheses of Spencer and Darwin. If it is true that, in the struggle for existence, animals have always perfected those capabilities which are most useful to them in defending themselves, and have gradually left behind, with the generations that have succumbed, all dispositions of the organism which were pernicious to the preservation of the species, why have they not succeeded in freeing themselves from trembling? Why, on the contrary, in critical, decisive moments, when danger confronts them, when their existence is threatened, when nothing is more imperative than flight, attack or defence, do we see animals paralysed with trembling, incapable of struggling, and perishing without their strength having in the least profited them? As Haller’s hypothesis is not sufficient to justify such a serious imperfection in organisms, we must seek the reasons and causes of this phenomenon elsewhere.
II
Charles Darwin, in his celebrated book on the 'Expression of the Emotions,’ says: 'Trembling, which is common to man and to many, or most, of the lower animals, is of no service, often of much disservice, and cannot have been at first acquired through the will, and then rendered habitual in association with any emotion.’[16] He then remarks that trembling is a very obscure phenomenon, and drops the subject.
Paolo Mantegazza accuses Darwin of great negligence with regard to this important problem, and says in his estimable work on 'Physiognomy and Mimicry,’ 'Darwin confesses that he does not consider trembling during fear useful; but, according to my experimental studies of pain, I find it most serviceable, as it tends to generate warmth, to heat the blood which is inclined to grow too cold under the influence of fear.’[17]
Since I must take part in this controversy, there is no other way but to examine attentively the various conditions of the organism in which trembling occurs, and then discuss the matter without prejudice. I acknowledge that I approach this task with some trepidation, because Mantegazza’s authority in physiology is so great that even Darwin’s name scarcely encourages an independent opinion. Let us consider the facts.
When we see horses, dogs, or men tremble from fear in the height of summer, at a temperature of 37°, and under the burning rays of the sun, one is inclined to think it is not from any necessity of warming themselves, all the more because monkeys, elephants, and many other animals always found at the equator tremble when they are frightened, even in their tropical countries.
In fever our teeth chatter while the temperature of the body is over 40°, and human economy, far from seeking to heat the blood by trembling, seems rather to stand in need of some mechanism which would cool it, so as to preserve life. After severe exertion or protracted work with the arms, our hands tremble, even when we are panting with heat. When exhausted after the forced marches which I had to make during my studies of fatigue, I have noticed that in the evening, on my return from the peak of Monte Viso or from the highest glaciers of Monte Rosa, my legs trembled, although the temperature of my body was one or two degrees above the normal. Tea, alcohol, coffee, and many stimulating medicines cause a very visible tremor. In convulsive laughter, pleasure, intoxication, voluptuous enjoyment, anger, when the necessity for heating the blood is certainly not apparent, one trembles also, the voice vibrates and the legs shake. All this makes it probable that Darwin is right, and more decidedly do I incline to his side when I think of the disastrous effects which trembling produces during fear. Seals and many animals, of which I shall speak more in detail in the chapter on Fright, tremble so violently that they cannot make their escape, and allow themselves to be overcome and miserably killed. How can we, amongst the sublime perfections which we admire in organisms, admit the contradiction that an animal, in order to warm itself, does not flee from danger but trembles till it is killed, whereas in running away, it could warm itself much better and save its life as well? But the question must not be judged in this way. The divergent opinions which arise in the interpretation of facts are most difficult to resolve in science, because to one of the adversaries there always remains a certain territory in which he may intrench himself.
III
In order to learn the actual nature of trembling we must first see how the muscles are made and how they act.