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Some of the lowest unicellular forms of animal life called Protozoa live more or less unprotected in water, though many have evolved various means of defence. Those unprotected unicellular animals which in the world's earlier history found their way into the digestive canal of higher animals in drinking water would, if they survived the action of the digestive juices (as many do), find themselves able to grow and multiply and live easier than those in the outside world, but they would have to produce a large number of spores, by the lowest form of reproduction, i.e., the asexual or simple division, and these spores would have to be more or less resisting to digestive juices, &c., in order to reproduce the species. In this way a class of animals represented now- a-days by the Sporozoa, i.e, animals which reproduce asexually by spores would arise. (They may also reproduce sexually at the present time);

But many members of the Sporozoa would probably find the action of the digestive juices very irritating, and would therefore be driven to seek further protection, e.g., such as is offered by living in the interior of a cell lining the digestive canal, which is exactly what some members of the Coccidiidae, viz.: Coccidium Oviforme in man and rabbits do at the present day, and this animal is known to reproduce itself asexually and sexually very much in the same way as the malarial germ does, except that it all takes place in the alimentary canal of its host, and that infection is by means of sub- stances swallowed.

Other members of the Coccidiidae, however, apparently passed either through the cells, or more likely between the cells, lining the digestive tract into the mucous mem- brane of that tract, which lies under those cells. There they would find new enemies waiting for them, viz. the so-called white blood cells or phagocytes, and to further protect themselves, they entered the capillary blood vessels and lay free in the blood stream just as the spores and the young malarial germ do to-day, but even here they would be liable to be attacked by the phagocyte, and to avoid them still further they entered the red blood cell where they found a haven of rest till the discovery of quinine. Those which did not find this haven would eventually die out in the struggle for existence. The unicellular animals which did this are to-day called Haemamoebidae and are known to cause disease in man, cattle, several birds, (pigeons, larks, sparrows, starlings, owls, screech owls) and in frogs.

Once in the red blood cell they circulate all over the body. But while these Haemamoebidae underwent their evolution in the tissues of warm-blooded animals, the gnats would have evolved and would have acquired their peculiar mouth apparatus for piercing tough structures and for sucking juices, and the females of certain of these gnats would have acquired the habit of taking in warm blood for the development of their eggs.

Consequently sooner or later the malarial parasite would find itself transferred from the tissues of the skin of man to the stomach of a mosquito. In this new habitat they would be liberated from the red blood cell (e.g., the present quartan and tertian Gametocytes) probably by their own action. On finding themselves unprotected in this new and probably not too congenial-abode, they would hasten to do what all unicellar animals try to do under such circumstances, and which their ancestors did in the human digestive tract-" Conjugated." The development of such motile treads as the male Spermatozoa would accelerate this act, as they are chemically attracted by the female cells and this might be highly beneficial if the juices in the stomach of the. mosquito were unfavourable, as, for example, they appear to be in culex and many other blood suckers.

The energy introduced by the male element into the female would induce the result of the union (the vermicule) to wander about to seek some protection from these irritating juices.

Consequently these vermicules would survive which passed between the cells of the stomach of the mosquito, even as their ancestors had done in the stomach and intes- tines of higher animals, but finding nothing much on the other side of these cells, (in- stead of a nutritious mucous membrane), they would, as other Protozoa in troubled times, become cucysted (Zygote) and break up into spores (Blastophores and Blastǝ) and these by the bursting of the capsule would be liberated into the cavity surrounding the stomach of the mosquito, and from thence into its various organs, including the salivary glands, and thence to its saliva and thence to man again.

The Haemamoeba of malaria in man develops in a mosquito called the anopheles, which is not very common, that of the bird in culex pipens which is common, and that of Texas fever in cattle in a tick. When once such a suitable course of life history is

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entered upon those germs which fail to carry it out die, .g., the germs of malarial fever in such culex die; consequently only one species is left (the most suitable for the life of the parasite) by means of which it can be propagated, viz. :—the anopheles mosquito.

In this way an explanation, perhaps not the true one, but still the explanation of the path of original entry into the human and of the original method of infection of the mosquito. Man must have been the original host, because in him the adult stage of the animal's life history is found the mosquito is only of importance in the sexual production.

Summary-By action of the digestive juice those Protozoa which enter the digestive tract of higher animals find themselves weeded out into those which can resist the juice with impunity and those which cannot. Those of the latter class which do not seek further protection die out, the others seek protection by getting under cover of the lin- ing cells of the digestive tract. Those which pass under the cells into the mucous mem- branes find new foes, to avoid which they enter the red blood cells, the gnat swallows the red blood cell. The parasite again finds itself in a digestive tract with juices and again protects itself by passing under the cells of the stomach and to reproduce its specics breaks up into fine spores. Only in one special species of mosquito or tick can a given parasite develop; in the others it dies.

Evolution of the Malarial Germ.

Unicellar Parasite in the digestive tract Protozoa.

Those which can resist the digestive Those which cannot comfortably

juice, eg., Amœba Coli.

Those which do not protect themselves-result :

extermination.

do so.

Those which do protect themselves.

Sporozoa.

Develop some chitinous protection and pass into

the cells, of the digestive tract Cocidiida Coccidium Oviforme.

ADDENDUM II.

I-Classification of the Malarial Parasites of Man.

Family Hamamosbi‹e—

Class.-Protozos.

Sub-Class.-Sporozon.

Order. Coccidiida.

}

Pass under the lining cells into the Mucosa and thence into the red blood cells.

Hænam bidæ,

In one stage of their existence they live in the blood corpuscles of some animal.

A. Genus I.-Hamamæbida.

Gametocytes are similar to Sporocytes.

Species A.-Hamamba Malaria Species B.-Hamamba Vivax...

B. Genus II-Hæmomenida.

Quartan Parasite.

Tertian Parasite.

Gametocytes dissimilar from Sporocytes. Species A.-Hæmomenas Precox

Tropicul Parasite.

+

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IL-Life History of the Malarial Parasite.

A.-Cycle of Golgi. ·

Asexual Reproduction.