SOME ASPECTS OF ZOOLOGY 37 



into the blood of the latter, and so sets up infection. Con- 

 versely, when an uninfected mosquito sucks blood from an 

 infected human subject, it sucks up the particular forms of 

 the parasite competent to start the new cycle in the mosquito's 

 body. Thus there is a remarkable adaptation of the parasite 

 to two very different animals, man and a mosquito, and its 

 life-history is correspondingly complex. But that is not all. 

 The same story might be told of birds and a form of mosquito 

 that we usually call the common gnat, Culex pipiens ; for 

 many birds suffer from a form of malaria, and the mode of 

 transmission and the life-history of the avian parasite are 

 extremely similar to those of the human parasite. But the 

 common gnat, as we all know, sucks human blood. Neverthe- 

 less, if infected with bird malaria, it cannot transmit it to man, 

 and if it draws blood from a malarious human subject, all the 

 parasitic stages of human malaria are promptly digested in 

 the gnat's stomach. Human malaria is only communicable 

 by gnats (or mosquitoes) of the sub-family Anophelinae, and 

 they are incapable of being infected by and therefore of 

 transmitting avian malaria. 



In other words, these two sets of parasites, so similar in 

 appearance, in structure and in mode of existence, are so 

 nicely adjusted to two different pairs of animals that they 

 occupy totally distinct provinces in Nature and cannot ex- 

 change places. A quite similar story might be told of the 

 microscopic parasites causing tsetse-fly disease in cattle and 

 sleeping sickness in man, and numerous other examples might 

 be cited. There is a further aspect of the phenomenon. The 

 Anopheline mosquito is so adjusted to the presence of the 

 malarial parasite that it does not seem to suffer any particular 

 inconvenience from its presence. As we say, it is " tolerant " 

 of the parasite. The constitution of the negro is similarly 

 adjusted, and he is tolerant of and suffers no grievous effects 

 from malaria, but the European, as we know well, is not so 

 adapted and suffers accordingly. 



We can pass to a much more familiar case of adaptation. 

 As all who collect butterflies and moths and rear up cater- 

 pillars know, most caterpillars are attached to a particular 



