being produced by ixodids of the first group, which attack only a single domestic animal and do not pass from one to another, consequently being unable to carry the virus. It is, then, alone due to the heredity of infection among arachnids that piroplasmosis is transmissible from a sick animal to a healthy one, and this heredity appears to us, therefore, as the necessary condition of the propagation of infection.

Piroplasmosis is the best type of hereditary diseases, in the proper sense of the word; that is to say, those which are transmissible from parents to offspring by means of infection of the eggs, an infection sufficiently limited, of course, not to arrest the development of the eggs, cause their death, and hence produce a parasitic castration. On the contrary, many of the diseases considered as hereditary-tuberculosis and syphilis, for example-are transmissible from mother to child only through an accidental lesion of the placenta, permitting passive passage of germs.

(c) Finally, a third point in the ixodian theory will detain us, namely, Under what form is the parasite transmitted by the ticks? Is there a simple inoculation of germs, as by a lancet, in the same form in which they have been received by the acarian, or rather is there a veritable development of the Plasmodia in the mosquitoes? This question is still one of those which it is impossible for us to answer, because in spite of the most assiduous efforts it has not yet been possible to find the least trace of piroplasms in the body of ticks. Quite recently, it is true, the celebrated German microbiologist Koch announced that he had seen "something," but the description which he gives is so vague that really nothing positive can be gathered from his communication.

Simple inoculation, that is, the mechanical transportation of the virus is perhaps possible in certain cases, notably for the ticks of the second and third groups, which transmit as nymphs, or as adults, the germs taken in in the preceding stage, but it is not probable as regards the ticks of the first group. Lounsbury, indeed, has shown that adult ticks, transferred from a sick dog to a healthy one, never transmit the disease. On the other hand, we have seen that the inoculation of piroplasms by the progeny of an infected tick is possible only in adult age. It is true that this is not altogether general. Theiler, confirmed by Laveran and Vallée, showed that bovine piroplasmosis and spirochaetosis are, in the Transvaal, inoculated by Rhipicephalus decoloratus, which are the progeny of infected mothers when they reach the larval stage. Further, the limitation of danger to the adult females may, as we have already shown, be explained by the fact that normally these females alone are parasites of domestic animals. Nevertheless, this peculiarity leads one to suppose that there is an evolution and permits one to ask whether in the course of its migra

tion through the different evolutionary stages of the ticks (egg, larva, nymph, and adult) the sporozoan does not itself undergo a series of transformations more or less comparable to those undergone by the paludic plasmodium in the body of the mosquito. Probably, then, this evolution can only reach the final stage (which is the formation of spores that are still hypothetical because they have not been seen) in the organism of the adult ticks. It could thus be explained why the larvæ and nymphs are incapable of inoculating the disease. They contain only the piroplasms at an intermediate stage, in which their inoculation into vertebrates would be insufficient to reproduce the affection.

Here, again, the hypothesis of a development is rendered more probable by comparison with the facts regarding the hæmogregarines of the tortoise shown by Siegel. As we have already said, the sporozoans perform a true evolution in the leeches with the formation of eggs and spores, and this scarcely leaves any further doubt regarding the reality of a similar evolution of the piroplasms in the body of the acarian.

To summarize, we ought, indeed, to recognize that it is still unknown how the young female ticks of infected mothers propagate the disease. It is probable that the spores exist in the salivary glands, but it has not been possible to establish this so far. As we said at the beginning of this chapter, the intimate mechanism of the ixodian transmission still remains mysterious in many respects. Nevertheless, there is not a single reason to deny this transmission itself, which is demonstrated by so many facts, as some still do.

Such is the real relation of the arthropods to pathology. Without doubt, unfortunately, the list of dangerous species is far from being complete, and the redoubtable faculty of propagating diseases does not belong exclusively to those animals in which it has been recognized thus far.

In closing, I would inquire whether the discoveries which I have recalled, and which have so great an interest from the point of view of pure science, have not already had some practical effect on prophylaxis. Fortunately, one is able to say that it is due to them that protection against diseases can be turned entirely in another direction than has been done. Owing to them, one can understand that, in order to be established on a rational basis, the combat against these affections should take note of two things; first, the destruction of biting arthropods wherever it is possible, and to as great an extent as possible; second, the protection of man and the domestic animals against the attacks of those which escape this extermination.

To give an idea of the results which can be obtained by the systematic practice of these two principles we will recall in a few words

what has been done against mosquitoes. In order to develop, these dipterous insects have absolute need of still, stagnant water. It is on the surface of these waters that the females lay their eggs and that the larvæ and nymphs live. The presence of quiet waters, such as lakes, swamps, ponds, pools, or puddles is necessary for the evolution of the Culicidæ and this necessity is felt by them all. Thus, it is well known that there are certain towns which have an abundance of water have also the doubtful advantage of being literally overrun by mosquitoes so that they are rendered almost uninhabitable. This is the case with Venice, Mantua, and Livourne. It is also well known that at Majunga, Madagascar, for example, which lacks fresh water, there are few mosquitoes and the place is very healthy. Nossy-Bé, on the contrary, possesses an abundance of fresh water and beautiful, luxuriant vegetation, but also an abundance of mosquitoes and constant malaria. It is also, for the same reason that in our great African island the presence of rice plantations causes a recurrence of fever. Soon after the rice is cut these plantations are flooded with stagnant water intended to hasten the decomposition of the roots. The whole region is then transformed into a veritable putrid marsh, very favorable to the development of mosquitoes. The insalubrity of rice culture has always been recognized and it is this which caused Vivarelli to say, "the rice plantations produce two things-rice and fever. The crop of rice may be deficient, but that of fever is always abundant."

Consequently, there is no doubt that the presence of standing water is indispensable to the development of mosquitoes, and it is no longer doubtful that in principle the exclusion of such waters results in the suppression of Culicidæ. This suppression is possible in a number of cases in which ponds, pools, wells, and reservoirs are unnecessary. When it is not possible because the reservoirs are necessary, the mosquitoes can still be destroyed by a simple process. It suffices to pour a little kerosene on the water. This substance kills the larvæ and nymphs as they come to the surface for respiration. The two measures which we have mentioned, the suppression of the stagnant waters and the use of kerosene, cause the extermination of the mosquitoes. They should be supplemented by a third, the protection of individuals against the bites of those that remain.

The Culicidæ are nocturnal insects which conceal themselves during the day and fly only after sunset. It is only necessary, then, to protect one's self against them during the night. This protection can be obtained mechanically by closing the openings of houses by means of metal screens sufficiently fine to exclude the insects, by covering the face by an ample veil attached to the hat, and by covering the hands with thick gloves. Wherever this triple means of protection is used thoroughly, the diseases caused by mosquitoes have disap

peared. Havana, which was overrun with yellow fever, is to-day healthful. Entire regions of Italy where malaria was endemic, are entirely freed from it. One can thus judge of the enormous progress realized by the prophylaxis resulting from this single zoological discovery, the rôle of mosquitoes in the propagation of diseases. It is true that equally brilliant results have not been obtained in connection with diseases transmitted by other insects, but the example of the mosquitoes justifies great hopes, and it is only proper to remember that the biological study of tsetse flies and fleas is hardly more than begun.

Much remains to be done in a branch of medicine where progress is necessarily slow, for it is specially concerned with exotic tropical diseases, which can only be well studied on the spot by the aid of missions, not only dangerous, but also very expensive.

In closing, I can affirm that the friends of science can never put too much money at the disposal of those students who devote themselves to the study of colonial medicine.

NATURAL RESISTANCE TO INFECTIOUS DISEASE AND

ITS REINFORCEMENT."

By SIMON FLEXNER, M. D.,
Rockefeller Institute for Medical Research.

Common observations early indicated that individuals of all animal species, and of the human species especially, were very unequally subject to disease. This elementary fact is impressed every day upon the thoughtful and has been, from the earliest times, the object of much ingenious speculation. Even to-day, and in spite of the acquisition of a wealth of new facts in physiology and pathology, we are not able to define fully the conditions that make for or against disease. However, the new knowledge which has been acquired enables us to see much more deeply and clearly into the complex mechanisms of disease than could be seen half a century ago; but unfortunately our insight has not been strengthened as regards all diseases, but almost exclusively in relation to the infectious diseases. In respect to the other class, or noninfectious or chronic diseases, among which are Bright's disease, vascular disease, malignant tumors, the gains in fundamental knowledge are far less great.

It may be axiomatic to state that all actual progress in unraveling the complicated conditions of disease depends upon precise knowledge of its underlying causes; and yet in an age in which comparative ignorance still requires that a certain amount of practice shall be empirical, it is well to bear in mind this notion, so that what is undertaken through knowledge may be kept distinct from what is adventured through ignorance. It has been to the lasting credit of the medical profession of an early period, when actual knowledge of the underlying causes of disease had not, and in the then state of development of the physical sciences could not, have yielded a single concrete fact, that one method-vaccination-and the most perfect one yet discovered of preventing a disease, and two drugs-quinine and mercury-specific for two other infectious diseases, should have been

Read at the university lectures on public health at Columbia University, New York City, March 1. Reprinted, by permission, from The Popular Science Monthly, July, 1909.