We have mentioned that the disappearance of migrated races which have journeyed to climates to which they cannot become acclimated by reason of physical characters which adjust them to colder and darker places, has been accomplished by the gradually increasing degeneration of successive generations. The basis of this condition is often, if not generally, a state of weakness of the parental nervous system which we call neurasthenia. This disease is so prevalent among white races in the tropics as to have merited the special term of tropical neurasthenia. It is also the basis on which other diseases are grafted and it deserves a further explanation, as it is perhaps the main reason why our relation to all our tropical dependencies must be commensal and not colonial.

It will be noticed that the climate in which a race underwent its recent evolution, exerts a tremendous influence upon the nervous system. The extreme differences between the various races make them immiscible. Cold and severe climates are the best for this evolution, because they cause a more intense struggle for existence, and the survival of the fittest is here the survival of the most active and intelligent, just as in the terribly severe glacial times only the most intelligent survived, and there occurred a rapid evolution of brain. In hot climates where exercise is distasteful, the struggle for existence is of a different type, and there is a survival of the least active and no general improvement in the race. Low tropical savages are the fittest for their environment, and the strenuous white man is the unfit.

Races which remained in cold, bracing climates, developed a nervous system fundamentally different from tropical man. It seems to be inherent in the nerve cells, constituting a higher type of tissue, capable of doing more and doing it better than the nerve tissues of the tropical creature - in other words a higher and therefore more complex machine. As we know so little of the composition of nerve tissue, it is, of course, the wildest kind of speculation to build theories, though we do know that a nerve system growing up and developing in cold, bracing air, protected from excessive light, nourished with good meat and plenty of food, actively exercised every day and rested every day, will not do its work in the tropics, but on the other hand will proceed to deteriorate. In blonds it simply burns up, for their temperature is generally higher than that of natives. Nervous exhaustion is then our great danger in the tropics, and its ravages are terrible.

Lombard * found that muscular power was markedly decreased in summer by several days of high temperature, especially with great humidity. Grijns has studied the reaction times of Europeans and Malays, and found that a sojourn in the tropics reduced the time fourteen and four-tenths per cent, as compared with newcomers, and sixteen per cent, as compared with that found in Europe. The Malay responded as well, if not better, than Europeans at home. He thinks that there is a general retardation of all nervous processes and the necessity for overcoming this inertia is responsible for the greater prevalence of neurasthenia in the tropics.* 

"Dr. Benjamin Ward Richardson found, after long experiment and practice, that (for white men) sixty-four degrees F. is the best temperature in which to conduct mental labor. If the temperature falls below this, the mind becomes drowsy and inactive; if it rises much above this, there is a relaxed state of the body and mind which soon leads to fatigue and exhaustion".

We have elsewhere stated that the white man has proved that he can live anywhere on earth, but as Ripley says, toleration of the climate does not mean adjustment to it. We must protect ourselves from light and from heat or cold, or in other words build up an environment around us like that at home as near as possible, and not live like the native. We must live in an oven in the arctics or cold storage in the tropics, or, as Benjamin Kidd says, like living in a diving bell: "It is a cardinal fact that in the tropics the white man lives and works under water . . . neither physically, morally nor politically can he be acclimatized in the tropics." He also shows that evolution in colder climates makes a far different machine than in hotter. Yet with all our care we cannot approximate our home environment, and there is then a strain for which we are not built, and though some stand it a long time, exhaustion comes sooner or later. The writer first called attention to tropical exhaustion in 1900, and since then the condition has been found almost universal, and is flippantly called Philippinitis. The symptoms are identically those of the same condition at home when due to living in a manner whereby the exhaustions of work, worry or illness are not repaired by food and rest. Man-son* says that "prolonged residence in hot countries causes vague, ill-defined conditions of debility".

* Journal of Physiology, 1892.

*  Archiv fur Anatomie unci Physiologie, 1902.

M. de Manaceine, in a work on "Sleep," states that when the temperature of a room is sixty-eight to sixty-six degrees many people sleep only six to eight hours; if sixty-four to sixty the same people sleep eight to ten hours, and if fifty-six to fifty-two, ten to twelve hours. In hot rooms, seventy degrees to seventy-six, they could sleep only three to five hours. In the tropics therefore, insufficient sleep is the rule, except where the nights are cool. This lack of recuperation must cause exhaustion, of which the latter insomnia is a mere symptom, and it is common in the Philippines in the hot cities, but not so frequent in the provinces where the nights are cool.

Another reason for the difficulty in recuperation in the tropics is found in the fact that heat dilates the superficial blood-vessels and lessens the amount of blood in the brain. We use a warm bath to cause cerebral anaemia and induce sleep in the restlessness of disease where there may be a cerebral congestion. It is especially beneficial in infants and children, and is the routine treatment in maniacal states. Hence, this same lessening of the cerebral circulation in the constant heat of the tropics must lessen its recuperative power.

* "Tropical Diseases," page 16.