REPRODUCTION AND THE LIFE CYCLE 199 



other stages as well are alike digested; hence the various species of 

 culex cannot transmit malaria to man. Similarly with other forms of 

 blood-dwelling parasites, each is apparently restricted to certain types 

 of hosts, although in some cases a certain latitude in this direction is 

 noted (Trypanosoma briicei, some species of Babesia, etc., may be 

 carried by different hosts). The ultimate explanation of this resist- 

 ance lies in the domain of physiological chemistry, and until this branch 

 of biological science is more fully worked up the full significance of 

 these adaptations will not be known. 



The same powers of adaptation that underlie the transmission of 

 malaria by mosquitoes apply to other cases of parasite transmission. 

 Mosquitoes carry trypanosomes from owl to owl; others (stegomyia) 

 carry the organism of yellow fever; tsetse flies (glossina) transmit 

 sleeping sickness in man or Nagana in cattle; other insects and ticks 

 carry different kinds of disease-causing organisms in lower domesti- 

 cated and wild animals; bedbugs transmit kala azar and relapsing 

 fever; while leeches are intermediate hosts for some parasites of fish 

 and amphibia. 



In many of these cases the parasites undergo a definite develop- 

 mental cycle in the body of the intermediate host, although in relatively 

 few cases have the happenings in such cases been fully determined. 

 In the case of malaria organisms, of Herpetomonas (Leishmanial 

 donovani and some trypanosomes, the most important phases in the 

 life history of the parasites, sexual reproduction whereby the vitality 

 is restored, are known to take place. In other cases, including the 

 majority of trypanosomes and spirochetes, and most other protozoan 

 disease-causing forms, little more than asexual multiplication within 

 the intermediate hosts is known to occur. 



It makes a very pretty subject for an academic debate whether 

 anopheles first gave malaria to man, or whether man gave acute 

 enteritis to the mosquito. There is some reason to believe that these 

 blood parasites, or at least some of them, have descended from the 

 coccidiidia, and that they have become specifically adapted for life 

 in the blood instead of in the epithelial cells of intestine or coelom. 

 The evidence for this is based partly upon the intracellular mode of 

 life characteristic of the majority of the hemosporidia and partly upon 

 Hintze's (questioned by Liihe on the ground of confusion with some 

 form of coccidiidia) observations upon the life history of the common 

 blood parasite of the frog, Lankesterella. While his observations have 

 been questioned, they have not yet been refuted, and his conclusions 

 are still possible, especially in consideration of the recent findings 

 of Miller ('OS) in the case of Hepatozooji peniiciosum (see p. 269). 

 Fertilization, according to Hintze, takes place in the intestine of the 

 frog, and the zygote moves like a gregarine through the fluids of the 

 digestive tract until it enters an epithelial cell, where it encysts. As 



