The inability to grow sufficient food is, of course, the reason why saturation points are low in tropical countries. The savage does not know how and must depend upon wild food. The evolution of cultivated plants and animals has been so gradual that they have adjusted themselves to bacterial and other enemies by evolving an immunity through natural selection, so that they can exist in large numbers and do not die of plagues. Transport these animals to a new climate where they meet new enemies against which they have not evolved immunity and they promptly die. Hence, a savage country can never have its grazing and other areas quickly stocked with imported domestic animals to the limit of its grass and grain production, for as soon as this is attempted we have rhindcrpest, surra, glanders, or some other plague, which wipes them all out of existence. This has happened time and time again in the Philippines, where there are millions of acres of magnificent ranges upon which countless herds of cattle could subsist if they could only resist the local infections. These ranges will not be occupied until there has been evolved by combined natural and artificial selection a breed of domestic animals with an immunity against these diseases. This, of course, will take many centuries, for it is an exceedingly slow process. Even the humped-back cattle called zebus, and the carabao imported from similar climates, have not yet developed this immunity, and are now and then destroyed over large areas. There are always many million acres of rice and corn land fallow because there are no carabao to drag plows and harrows. Hence, we see that these islands, as well as all other lands which, until a few centuries ago, had few or none of our food animals, cannot now support anywhere near the population which could be supported if all the land were used which is capable of use for cultivation or grazing, but the development of the proper species will slowly raise the saturation point for many centuries.
It might be said that through the use of vaccines and serums an artificial immunity could be conferred on domestic animals, but this implies that men of much intelligence must be on the spot to do it, for the native has not the requisite brain. For this purpose alone there will be required a higher percentage of white men in the tropics than our past experience has shown could be sustained, or perhaps could of themselves stand the climate. The present proprietors of lands which are fallow for the want of draught animals, have been asked why they did not import more and go to work, and they have promptly replied that it was of no use, that they had done so before, but that the cattle had at once died of rhinderpest. Until we learn how to prevent this, there must be great loss by overstocking ranges, which at present are supporting as food for the natives the maximum number of deer and wild hog of which they are capable. These animals may not be immune, but probably are; anyhow, they are so scattered that infection can spread from one to another with difficulty, if at all.
When we went to the Philippines we were told that European stock could not live there - it had been tried time and time again. But we were arrogant, and knew better. Some cavalrymen said it was nonsense, and that the English did not know how.
Nevertheless, our informants were correct. At the present writing, to save the remnant of our stock, we are occupying our time killing thousands of horses and mules infected with surra - a disease to which the Indian native and domestic animals have at least a partial immunity, gained through several mil-leniums of selection, but to which our stock is not immune. So the increase of population must be very slow to keep pace with the inhabitant's ability to increase the food output.
Finally, the filthy habits of savages prevent existence in dense masses on account of self-poisoning and infection. It reacts also upon their food supplies, for their domestic animals die of diseases easily avoidable by attention to simple cleanliness. The Filipinos raise to maturity only a very small proportion of the young hogs and chickens, there being great mortality from numerous infections grouped as chicken cholera and hog cholera. These animals, by the way, have been in the Philippines for many centuries, and were imported from equally filthy countries, and have evolved a partial immunity, so that they are not entirely wiped out, as is the case with more recent importations of domestic animals from clean countries. It was a curious result of our sanitary efforts to clean up the towns during cholera times, that the natives did not thank us in the least for having saved them from cholera - not at all - on the contrary they considered it all foolishness. There was no cholera, they said, because nearly all would have died, as in prior epidemics. But they did notice that they were able, under clean conditions, to raise nearly all of their young chickens and hogs, and they thought, therefore, it was a good thing to be more sanitary - not that it saved human lives but that it saved more hogs. Nevertheless, as soon as we had to relax our efforts, when our military control ceased, they relapsed into old ways and were as filthy as ever, and cholera has repeatedly visited them. To survive they must be under military control.
Culture may even reduce the saturation point after increasing it. According to a manuscript in the library of the Marquis of Lansdowne, the census of Ireland, taken in 1659, showed only 500,000 people, but in the next century and a half it had increased to 8,000,000, most of whom lived in abject poverty. They found that they could obtain a better living by migrating, and the population has steadily gone down until it is now reduced about half. The survivors would never be content to live as their ancestors, so that, though the amount of food produced is probably more, there is a greater share for each person. The primary causes of this depopulation will be taken up later.