This brings us face to face with one of the many problems connected with the subject of immunity which so far have successfully eluded all attempts made to solve them. Experience has shown repeatedly that although _artificially_ acquired immunity from a particular poison can be handed on by means of an animal's serum, yet the _natural_ immunity from a given poison enjoyed by one species of animal cannot be similarly transferred to less-favoured varieties. This fact has long been recognised in the case of poisons of bacterial origin. Thus, white rats are absolutely immune from diphtheria, but Wassermann showed some years ago that the serum of these animals has no power whatever to counteract the action of diphtheria-toxin in other animals. Guinea-pigs were inoculated with fatal doses of diphtheria toxin along with white-rat serum; but although other guinea-pigs treated with the same toxin mixed with the ordinary artificially elaborated anti-diphtheritic serum survived, those which received the rat serum died in every case. Now very similar results have been obtained by Calmette in respect to the serum of animals naturally immune from serpent venom. The serum of the refractory little mongoose, as well as that of the hedgehog, is wholly unable to save other animals from the lethal effect of venom poison, and similar results have been noted in respect to swine serum. But a very curious fact has also been discovered by Calmette--_i.e._ that these so-called naturally immune animals very frequently are quite incapable of being artificially trained to elaborate a serum possessing protective powers which can be transferred to another animal. How splendid a domain for beneficent research lies before the scientific investigator is apparent to all, and the important work already accomplished is but an augury of yet greater discoveries awaiting the labours of such leaders as Calmette. It is not surprising, therefore, that the scientific interest in toxins and anti-toxins shows no signs of abatement. On the contrary, the competition for obtaining and working the new "claims" which pioneer research enthusiasts are constantly engaged in "pegging out" remains as keen as ever. Despite, however, the extraordinary interest which this subject has aroused in scientific circles all over the world, nearly ten years elapsed before any notice was taken of the curious discovery made by two brothers that the blood of eels contained a highly poisonous principle, and the memoir containing this remarkable announcement remained until comparatively recently buried in the Italian journals where it was first published. Calmette was, we believe, the first to call attention to this discovery of the brothers Mosso and give it the prominence it deserves, and both he and other investigators have not only fully confirmed it, but have greatly added to our knowledge concerning the character of the poison contained in eel serum.