causing Disease in Man in Nyasaland. 



119 



Chart 4. — Curve representing the Distribution, by Percentages, in respect to Length, of 

 500 Individuals of the Trypanosorne of Naturally Infected Dog, Strain I, Dog 48, 

 taken on nine consecutive days from Rat 2471, after passage through rats for two 

 years. 







M i 



C 



r o n s 





17 



18 



19 



20 



21 



22 



25 



24 



25 



26 



27 



28 



29 



30 



31 



32 



35 



34 



35 



36 



37 



38 



19 















































18 















































17 















































16 















































15 















































14 















































05 13 















































Q) 



JDO 12 















































r6 „ 















































+-> 



c 10 

 oi 9 



























































































>- s 















































o> 7 















































°- 6 















































5 













































4. 















































3 















































2 















































/ 

























































































The curve is now practically monomorpbic. The short and stumpy and 

 the intermediate forms have almost disappeared, and only the long and 

 slender survive. But, it may be objected, perhaps this curve from Eat 2471 

 is a mere accident due to some peculiarity in the rat ; it is possible that if 

 another rat is inoculated from it the curve will be found to be as dimorphic 

 in type as that of Eat 1218 on Chart 3. That this is not so will be seen 

 by the following chart, which represents the gradual change in type which 

 takes place in this trypanosorne by passage through rats. The first rat, 67, 

 was inoculated from the original dog ; the second rat, 312, was inoculated 

 from Eat 67 ; Eat 407 from Eat 312, and so on through a series of 20 rats. 

 The unbroken line represents the percentage of the long and slender forms, 

 the broken line the short and stumpy. For example, Eat 67 has 30 per 

 cent, long and slender and 70 per cent, short and stumpy. In Eat 670 the 

 long and short forms are almost equally divided ; Eat 786, 82 per cent, long 

 and 18 per cent, short. The percentage of the long and slender gradually 

 increases until at the end of 17 passages it reaches 100 per cent., so that 



