268 



NATURE 



[November 5, 19 14 



parallel column attacking from quite a different direc- 

 tion circumvented the difficulty in this respect. 



The story commences with the studies of those 

 obscure and often despised workers who toiled in the 

 domains of parasitology. Some of the largest para- 

 sites were known in antiquity ; but the ancients 

 possessed quite a wrong notion of their origin, which 

 they attributed to spontaneous generation — that is, 

 they thought the parasites originate de novo in the 

 host. In the seventeenth century, however, Radi 

 proved that this hypothesis does not hold for certain 

 insects ; and later Pallas argued that parasites 

 originate ab ova, like other animals — that is, that 

 their eggs escape from one host and enter another 

 host, thus leading directly to the presence of parasites 

 in the latter. This history possibly still holds for 

 certain parasites ; but in 1790 Abildgaard showed by 

 experiments that some parasites of fish live not only 

 in those fish, but for a part of their existence in 

 certain water-fowl ; and this extraordinary law, which 

 may be called after De Barry's term slightly changed, 

 the law of metaxeny, was proved during the middle of 

 last century by Eschricht, Steenstrup, and especially 

 Kiichenmeister, to apply to a large number of Platodes 

 and Cestodes. Afterwards Leuckart, Melnikoff, and 

 others extended the law to cover other species, includ- 

 ing species of Nematodes. A most important case was 

 that of the Filaria niedinensis, the famous guinea 

 worm of man, which was shown by Fedschenko in 

 1869, following a suggestion of Leuckart, to be in 

 common with man and a water flea (Cyclops). All 

 this constituted a discovery which was both remark- 

 able in that it exhibited the wonderful devices of 

 nature for propagating parasites from host to host, 

 and was also of the highest importance to mankind 

 (though few recognised this point at the time) because 

 it showed us how many of our great diseases are 

 likely to be acquired. 



Let me dwell on this point for a moment. Para- 

 sites, accustomed as they are to dwell in the safe 

 retreats of certain portions of their host's body, must 

 be exposed to great dangers whenever they come to 

 pass, as they must do from one individual host to 

 another. Thus if this passage is effected merely by 

 the egg, it is obvious that the eggs must be poured 

 out in immense numbers to compensate for their 

 immense destruction outside the body of the host ; 

 since it would alwa3S be probable that only a very 

 minute proportion of the eggs would ever find their 

 way again into fresh hosts of the proper species. In 

 order to avoid these difficulties, nature, I presume 

 through an infinite period of evolution, has enabled 

 many parasites to acquire a more safe and certain 

 route of entry — through other animals which are asso- 

 ciated frequently with their first species of host. Thus, 

 for instance, a parasite of dogs and cats develops also 

 in the dog-louse, and is therefore not scattered 

 broadcast from the dog and the cat over the surface 

 of the earth, but is retained alive in close proximity 

 to the bodies of those anim.als. Remember that 

 nature is as solicitous for parasites as for the higher 

 animals which contain them. She thinks no more 

 of man than of the minute germ which infests him. 

 From her open hand fall innumerable species, and she 

 endows them all with equal powers of looking after 

 themselves and their own interests. It is not in the 

 interest of parasites to be wasted abroad. It is in 

 their interest to be brought again as quickly as pos- 

 sible to the host in which evolution has formed them 

 to live — and that is why this wonderful law of 

 metaxeny holds good. It must be remembered, of 

 course, that no species of parasite should be allowed 

 to develop in excessive numbers in the same individual 

 host, because this would result in the death of the 

 latter, and therefore also of the former. The parasite 

 NO. 2349, VOL. 94] 



must therefore find some means of transference at 

 some stage in its life-history ; and that route must be 

 the most expeditious possible. This most expeditious 

 route will often be, as I have said, through another 

 species of animal connected in its habits with the 

 original host. 



Following upon the discovery of Fedschenko, Man- 

 son in 1877 showed that the embryos of another Filaria 

 of man {Filaria hancrofii) develop in a species of 

 mosquito, probably a Culex. The life-cycle of this 

 parasite, up to the point to which he demonstrated it, 

 was' closely similar to that demonstrated by Fed- 

 schenko for F. medinensis ; and Manson did not com- 

 plete the story. Now, however, other important dis- 

 coveries w-ere made, which showed that animal 

 parasites do not by any means belong only to the 

 zoological groups of the Cestodes, Platodes, and 

 Nematodes, but that many of them are unicellular. 

 Thus Leuckart established' the protozoal order of the 

 Sporozoa; Lankester and others discovered Trypano- 

 somes and Haemogregarines ; Davaine discovered Cerco- 

 monads, and Losch discovered the group of parasitic 

 Amoebae. Lastly, in 1890, Laveran made the most 

 important discovery that malaria is associated with a 

 minute protozoal parasite of the blood; and his ob- 

 servations were followed by those of Danilewski and 

 others, who showed that similar parasites are to be 

 found in many animals. All this extended the cata- 

 logue of animal parasites over a vastly larger field 

 than was known to the ancients; and the morphology 

 and life-history of the parasites in the known hosts 

 were elaborated by numbers of workers, who also 

 succeeded gradually in connecting them with some of 

 the most important diseases which inflict man and 

 ainimals. Thus it was gradually shown that Amoebae 

 are the cause of one form of dysentery and abscess of 

 the liver; that Trypanosomes cause the deadly nagana 

 of domestic animals in Africa, and the equally deadly 

 sleeping sickness of man, besides many other maladies 

 of animals of numbers of species. Ankylostomes were 

 proved to cause miner's anaemia, which also was 

 shown to be prevalent through vast areas of the 

 tropics, by no means only among miners. The 

 Trematodes called Bilharzia were shown to produce 

 a terrible form of cystitis in Egypt and elsewhere. 

 F. bancrofti were proved to produce elephantiasis and 

 a number of other maladies which are very common 

 in the tropics. Simultaneously malaria was seen to 

 be caused by at least three different species of the 

 group discovered b}^ Laveran. 



At the same tirne, of course, as I have said, the 

 habits of all these parasites in the hosts in which they 

 had been found were being minutely examined. But 

 up to the last decade of last century we still could form 

 scarcelv any definite idea as to how the protozoal 

 parasites pass from one individual host to another. 

 The law of metaxeny which had been proved to apply 

 to many of the larger parasites had not been extended 

 to the 'smaller ones. In 1889, however. Smith and 

 Kilborne discovered a small parasite called Piroplasma 

 in the blood of cattle suffering from Texas fever ; and, 

 more than that, showed that in some mysterious way 

 the infection is carried from ox to ox by means of 

 certain cattle ticks — though they did not demonstrate 

 in any wav that these parasites undergo a metaxynous 

 stage" of development in the ticks, and indeed failed 

 to find them at all in these arthropods. In 1896 also 

 Bruce made his famous discovery that the Trvpano- 

 somes of nagana are conveyed by certain tsetse-flies, 

 but supposed that the carriage is a mechanical one. 

 And there the matter rested until the solution of the 

 malaria problem opened a new field. 



We now turn to the subject of malaria. Economic- 

 ally, as well as medicallv, it is certainly the most 

 im'portant disease in the tropics, perhaps in the work!. 



