726 



SCIENCE 



[N. S. Vol. XXXVIII. No. 986 



we send for Sir Almroth Wright to pacify 

 or chastize them with his vaccines. 



So that, as Darwin said, "An organic 

 being is a microcosm, a little universe, 

 formed of a host of self-propagating organ- 

 isms, inconceivably minute and numerous 

 as the stars in heaven" — as we ourselves 

 are but parts of life at large. 



The three main functions of blood are: 

 that it is a means of respiration, a means 

 of nutrition and a defense against invading 

 organisms. 



And now to these latter. A blood para- 

 site proper is a living being, vegetable or 

 animal, passing part or the whole of its 

 existence in the blood of another living 

 being, upon which it lives, this being ob- 

 ligatory and necessary to its life-cycle. 



It was in 1841 that the first blood para- 

 site was seen by Valentin in the blood of a 

 fish, and two years later Gruby gave the 

 name Trypanosoma to an organism he 

 found in the blood of a frog. But since 

 Laveran's discovery of the malarial para- 

 site in 1880, we have learned to differen- 

 tiate many other parasites as causal agents 

 of such diseases as I shall mention later in 

 connection with the various parasites. But 

 we know as yet dangerously little about 

 most of them, so that we have strenuously 

 to resist the temptation to make our ac- 

 count of them sound too harmonious, be- 

 fore we have found half the notes of the 

 chord we are trying to play. "We speak, as 

 it were, with authorized uncertainty, and 

 there are parts of our science which, after 

 all, are only expressions for our ignorance 

 of our. own ignorance. These parasites 

 have a very complicated life-history; part 

 of their life-cycle is passed in the blood of 

 man or beast, and part in various parts of 

 the body of some blood-sucking inverte- 

 brate, such as a fly, mosquito or tick, which 

 transfers the parasite to another animal 

 whilst feeding from him. It was thought 



formerly that blood parasites would be a 

 restricted order, but the work of recent 

 years has shown that they have an enor- 

 mous distribution both geographically and 

 as regards their hosts. For instance, dur- 

 ing the last five years I have had the op- 

 portunity of examining all the animals (in 

 the large sense of the word) which have 

 died in the Zoological Gardens. I have 

 examined the blood of over 8,000 animals, 

 coming from all parts of the world, and I 

 have found parasites in the blood of 587 

 of them, that is, in about 7 per cent., and 

 in 295 species of animals I have found 

 them for the first time. I mention this 

 just to give you some numerical idea of 

 their occurrence and distribution. 



It will- be better to take first those para- 

 sites which live in the plasma, and then 

 those that live in the corpuscles, rather 

 than to attempt to take them in their, at 

 present rather uncertain, biological order; 

 and I will begin at the bottom, biologically 

 speaking, that is, with the bacteria which 

 are plants. These only require mention, 

 since they do not live in the blood as para- 

 sites proper, but only as accidental para- 

 sites — that is, parasitism is not necessary 

 to their life-cycle; they get into the blood 

 in the later, or in certain, stages of certain 

 diseases. 



An example is the blood of a Senegal 

 turtle-dove which died in twenty-six hours 

 from fowl cholera. This bacillus was dis- 

 covered by Pasteur, and is interesting, as 

 it was his work upon it which led to his 

 discovery of the attenuation of a virus, and 

 of its transformation thereby into a pro- 

 tective vaccine. 



The first parasites proper I shall men- 

 tion are the spirochetes. These have at 

 present rather an insecure position in our 

 idea of nature ; they were formerly classed 

 close to the bacteria, but now they are 

 placed tentatively among animals, and 



