MANKIND AND THE INSECT 325 



repeats the process of feeding, growth and spore-forma- 

 tion, the patient soon becomes greatly enfeebled. It is 

 known, too, that the recurrence of chills and fevers is 

 simultaneous with the successive generations of spores 

 (together with the poisonous waste products) which are 

 discharged into the blood. Quinine regularly adminis- 

 tered kills the parasites ; but they are very difficult to 

 eradicate, and may remain in the blood for years, causing 

 intermittent fever. Nevertheless, the process of asexual 

 reproduction cannot go on indefinitely. A time comes 

 when the disease exhausts itself, and spore reproduction 

 ceases. Rejuvenation is possible, but only as the result 

 of sexual conjugation ; and for some inscrutable reason 

 this cannot be effected in human blood. This fact was 

 suspected prior to Ross's epoch-marking discovery, for 

 other observers (notably Sir Patrick Manson) had worked 

 out the life-cycle of another parasite in the blood of birds, 

 which is sucked up by certain kinds of gnats, in whose 

 bodies its sexual stage is passed. Indeed, it was a sug- 

 gestion made by Manson that the " intermediate host ' 

 of the malaria parasite might prove to be a gnat or mos- 

 quito (the terms being synonymous) which first induced 

 Ross to enter upon his investigations. He first experi- 

 mented with common grey gnats of the genus Culex ; 

 but he found that the parasites sucked up by them with 

 the blood of malarious patients were simply destroyed 

 and digested in the insect's stomach. After some difficulty 

 and delay, he proceeded to experiment with gnats (or 

 mosquitoes) of a different kind — members of the genus 

 Anopheles ; and with these he was successful. 



It has since been shown that throughout the world 

 these particular gnats of the genus Anopheles are indis- 

 pensable for the incubation and spread of the malaria 

 parasite. What really happens may be briefly summed 



