300 SOME MINUTE ANIMAL PARASITES 



Parasitism in the past affords scope for many 

 interesting speculations. Time was when the earth 

 was populated with enormous mammals, huge flying 

 reptiles, giant primitive birds, and large Amphibia. 

 All now have vanished, and their fossilized remains 

 or impressions are all that persist to tell the secret 

 history of the "giant" era. To what causes can 

 the wholesale disappearance of such a wonderful 

 fauna be ascribed ? Physical changes in the earth 

 alone are insufficient to account for the vanishing of 

 all these animals. The evolution of chemical fumes 

 would tend to kill all forms of life at about the same 

 time, and there is no good evidence of this. One 

 recent suggestion is that the disappearance of these 

 mammoth creatures from the earth's surface was 

 due to the activity of malarial parasites, trypano- 

 somes, or other parasitic Protozoa. When we 

 realize that within the last century human tribes 

 have almost disappeared as the result of the action 

 of parasitic Protozoa, the hypothesis does not seem 

 entirely unlikely. Reverting to the opinion that 

 the newer the parasite is to its host the more deadly 

 it is, would it not be likely that the intensity of 

 virulence in those early days would be enormous in 

 comparison with what it is now ? The fate of 

 horses and cattle in the "fly belt" of South Africa 

 is a striking example of the terrible effects of trypano- 

 somes in causing the disappearance of fauna in a 

 short period ; and in the dark ages of the past, when 

 parasitism was probably newer and the field was 

 wider, it must have been infinitely more drastic than 

 at the present time. 



