The life and work of Elie Metchnikoff are so intimately bound together that, in a biography, it is impossible to separate them. That is why the description of his work necessarily has been dispersed along the story of his life; but, just as, in order to judge of a work of art, one has to draw back and contemplate the whole, we must also, after following the evolution and successive stages of E. Metchnikoff’s scientific works, take a full view of his work as a whole.

He was a born biologist; everything connected with life interested him. In his childhood, he observed plants and animals. At the age of fifteen, he became acquainted with microscopic beings; they aroused in him such powerful interest towards the primitive forms of life that, from that moment, not only his future path was marked out for him but also his method of starting from the simple to elucidate the complex. He was imbued with Darwin’s theory of evolution; having begun by the study of inferior animals, he began to look for their connections with other groups.

He endeavoured to establish the continuity and the unity of phenomena in all living beings. According to his method of studying first what was simplest, he turned to embryology, for in the egg and the embryo it is possible to follow step by step the transformation of the simple to the complex and to see the origin and development of all the constituent parts of the organism. Moreover, the embryo is exempt from secondary complications, due to the multiple external conditions of post-embryonic life.

Metchnikoff was able to establish, from embryological data, that the development of lower animals takes place according to the same plan and under the same laws as that of higher animals. In all of them, the segmentation of the egg is followed by the formation of embryonic layers, of which each gives birth to cells and to definite organs. Superior forms repeat, in their embryonic life, the evolution cycle of inferior forms.[37]

This common plan in the embryology of all animals established their genealogical continuity and strengthened the Darwinian theory.

Metchnikoff’s studies, carried out on the various groups of animals, contributed towards the foundation of comparative embryology. Owing to the comparative method, he had made himself familiar not only with the morphological and functional continuity of divers organisms, but also with that of their constituting cells; a comparison between the latter and unicellular beings was inevitable. That is why, having ascertained that the mobile cells of the lower Metazoa absorbed foreign bodies by inclusion, he naturally concluded that that phenomenon was similar to digestion in unicellular beings.

Having established the fact of intracellular digestion in lower animals, he extended it to certain cells of the higher animals; thus his phagocyte theory was born.

Seeing that unicellular beings, like the mobile cells of Metazoa, englobe, not only food, but foreign bodies, he asked himself whether this was not at the same time a defensive action. Such a possibility brought no surprise to a zoologist, accustomed to see that, in the struggle for existence, animals often devoured their enemies.

All the materials for the building up of the phagocyte theory were therefore ready in Metchnikoff’s mind when he asked himself, as by an intuition, whether the white globules of our blood, globules so similar to amoebÆ, do not play the part of a defensive army in our organism when they envelope in accumulated masses intrusive bodies injurious to the organism.

The thought was but the result of a preparatory work already accomplished; it was the butterfly escaping out of the chrysalis.

Metchnikoff had recourse to his method of simplification in order to solve the question.

The organism of the higher animals being extremely complicated, he went down as far as the transparent larva of the starfish (bipinnaria) in order to watch with his own eyes the phenomena which take place within it. He introduced a rose-thorn into the transparent body of the larva, and noted the next day that the mobile cells in the latter had crowded towards the splinter, like an army rushing to meet a foe.

The analogy of this phenomenon with inflammation and the formation of an abscess was striking. Metchnikoff said to himself that since most diseases in the higher animals are accompanied by inflammation and provoked by microbes, it was chiefly against these microbes that our defensive cells had to struggle. He named the defensive cells phagocytes.

He confirmed his hypothesis by another observation, equally simple. In a little transparent crustacean (Daphnia) infected by a small parasitic fungus, (Monospora bicuspidata), he was easily able to observe the struggle between the animal’s mobile cells and its parasites.

These two simple observations served as foundation and supports to the bridge by which Metchnikoff connected normal biology with pathological biology. Having entered the domain of the latter, he studied various microbian diseases, and asked himself why the organism was sometimes liable and sometimes refractory. In order to elucidate this question, he turned again to lower animals, in which he could easily observe the most intimate phenomena, simplified.

He ascertained that liability in an animal corresponded with the fact that microbes introduced into the organism remained free and invaded it, whilst immunity coincided with the inclusion and digestion of the microbes by phagocytes.

He also found that, in artificial immunity, the phagocytes are accustomed gradually, by preventive inoculations, to digest microbes and their toxins.

Thus he established the fact that phagocytosis and inflammation are curative means employed by the organism.

All his ulterior researches, his studies on the various categories of phagocytes and their properties, on their digestive liquids, on the formation of antitoxins, on the different properties acquired by the blood, etc., were but the natural development of those premises.

He had proved that the part played by the phagocytes consists, not only in the struggle against microbes and their poisons, but also in the destruction of all the mortified or enfeebled cells of the organism, and that atrophies are nothing more than the absorption of cellular elements by the phagocytes.

He found that senile atrophies have the same cause, and asked why the cells of old people’s organisms should become enfeebled.

He demonstrated that the principal cause is the chronic poisoning of the cells by toxins manufactured by microbes in the intestine. Premature senility was the result—a phenomenon as pathological as any disease.

The source of the evil, therefore, resides in the intestinal flora. Accordingly he started to study the latter, as also senility, in order to find means of struggling against both.

His researches enabled him to indicate a series of means, based, on the one hand, on the struggle against microbes, and, on the other, on the defence of the noble cells against destructive ones.[38]

The study of old age led him to that of syphilis, a disease which provokes an arterio-sclerosis which is similar to that of old people; the study of the normal intestinal flora was followed by that of intestinal diseases, such as typhoid fever and infantile cholera.

Finally, he progressed towards the last phenomenon, the most mysterious in nature, Death.

Researches on the silk-worm moth—a rare example of an animal the life of which ends in natural death—allowed him to conclude that the latter is due to an auto-intoxication of the organism.

But he only just raised the veil of the great mystery; it was his last work....

Metchnikoff’s philosophical evolution ran on parallel lines with his scientific researches.

When studying the laws and the unity of vital phenomena he found that their harmony was occasionally broken by the collision of internal conditions with the environment and that regrettable consequences ensued. He saw an example of that in human nature, full of disharmonies due to its animal origin.

These considerations caused the pessimism of his youth. But his energetic, pugnacious temperament could not remain content with a passive acceptance of facts.

He started to study the lack of harmony in human nature and its causes, and sought for means to combat these causes. Gradually he reached the conclusion that the greatest human disharmonies are provoked by the rupture of the normal cycle of our life, by the precocity of senility and of death, chiefly arising from a chronic poisoning by the toxins of intestinal microbes.

But having acquired the conviction that it is possible to struggle against that intoxication, he concluded that science, which has already done so much to fight diseases, would also find means of struggling against premature old age and precocious death, thus leading us to the normal vital cycle, orthobiosis.

Then disharmony, transformed into harmony, will cause the greatest of ills to disappear.

Faith in the power of Science and in the possibility of modifying human nature itself through Science was the foundation of the optimistic philosophy of his maturity. Thoughts full of strength and hope shine like leading stars all along his philosophical works.

“Alone, Rational Science is capable of showing humanity the true path.”

“The real goal of human existence consists in an active life in conformity with individual capacity; in a life prolonged until the appearance of the death-instinct, and until Man, satisfied with the duration of his existence, feels the desire for annihilation.”

“Man is capable of great works; that is why it is desirable that he should modify human nature and transform its disharmonies into harmonies.”

“If an ideal capable of uniting men in a sort of religion is possible, it can only be founded on scientific principles. And, if it is true, as is often affirmed, that man cannot live without faith, it must be faith in the power of Science.”

Thus Elie Metchnikoff had begun by the study of nascent life in inferior beings; by a logical and continuous chain, he had followed the whole cycle of development of living beings in their continuity and their whole.

From the initial question of intracellular digestion he had reached the most exalted problems which can occupy our minds, the harmonising of human discords through knowledge and will.

Such is the harmonious edifice which he has built.

No vital question was indifferent to him. He tackled the most difficult and most mysterious among them with courage, moved by an invincible impulse towards Truth and sustained by enthusiasm and faith in the power of Science.

The beauty of a work of art consists in the harmony and unity of a realised conception.

Thus a Gothic cathedral, by its graceful and harmonious lines, expresses an impulse towards higher spheres; it leans solidly on the earth only in order to soar better towards the heavens.

Such is also the character of Elie Metchnikoff’s life-work.