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. [16] Darwin, chap. iii., p. 67. [17] Mantegazza, chap. vii., p. 119. 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.