Karl Pearson 
119 
trypanosomes taken from a single species of host. But how far the resemblance 
in these cases is produced by a selective influence of the host and not necessarily 
by an identity of all the members of the strain before transference to the host is 
not demonstrated. 
On the other hand while divergence due to host will account for the divergences 
which are so notable in T. pecorum, it will not account for the divergences in the 
human strains; these are startliugly conspicuous even if we confine our attention 
to a single species of host. Precisely the same remarks apply to the trypanosomes 
similar to those causing disease in human beings found in wild game and in the 
tsetse fly itself. There must be another source for these divergences. 
(7) Discussion of the Heterogeneity luhich is statistically demonstrable in the 
hulk of the Trypanosome Measurements. 
The reader who has attentively followed the course of the argument in the 
previous sections will be prepared for the next step in this memoir, the attempt to 
account for the large divergences between strains of trypanosomes in individuals 
of the same species by the heterogeneity of those strains. My suggestion is that 
the strain in one fly differs from that in another because the components do not 
appear in the same proportion, the strain in one specimen of wild game from that 
in another, or in one man from that in another because they have been bitten by 
a fly containing the components in unlike proportions. The host does make some 
difference, either by nutrition or selection of trypanosomes, but it is a minor differ- 
ence. Thus consider what we may probably hold to be pure strains and observe 
the average differences in length found by Sir David Bruce and his colleagues: 
Microns. 
T. simiae* 
T. caprae t 
T. pecorum 
Mcera Cattle J 
Wild G. iiii)i:<:ita}iii^ 
Goat 17-3 
Monkey 18-1 
Waterbuck 26-8 
Ox 25-7 
Goat 25-3 
Sheep 2.5-6 
Donkey 13 '5 
Ox 14-2 
Goat 13-8 
Dog 13-8 
Rat 14-8 
Goat 13-5 
Monkey 13-6 
Dog 14-2 
Guinea Pig 14 '6 
Rat 14-0 
Max. Difference 0-8 Max. Difference 1-5 
Max. Difference 1'3 
Max. Difference 1 '1 
We may thus anticipate that in a pure strain the change of host would hardly 
make a difference of more than 2 microns in the average length. We must 
* E. S. Proc. Vol. 85, B, p. 479. 
t E. S. Proc. Vol. 86, B, p. 279. 
t B. S. Proc. Vol. 87, B, p. 3. 
§ Ii. S. Proc. Vol. 87, B, p. 10. 
