Karl Pearson 
105 
If we turn to the Mvera cattle, we find there were four sources of trypanosomes 
for the ox, ttuo for the goat, and the same two for the dog — these two sources being 
two of the four cattle sources. There was only one source for the rat, but I have 
not discovered how far it was identical with one of those for ox or goat*. In the 
Mzimba donkey strain there was one source for dog and rat. In the wild-game 
strain there were, I make out, eight sources of trypanosomes for the goat, four for 
the dog, and only one for the ratf. 
Thus the individuality, which might be supposed to influence the result, 
because we are treating of trypanosomes in this case from a single rat, in the 
Mvera cattle case from a single rat, and in the Mzimba data from only two rats, 
may really arise from the fact that the rat strains in each case are derived from a 
single source, while the dog, goat and ox strains show a multiplicity of sources. 
The troublesome point is that the experimental part of the work has not been 
designed to answer what seem to me fundamental questions. We cannot directly 
inquire what difference the host makes because different hosts have rarely been 
treated with the strain from a unique source. We can say that dog and goat are 
interchangeable for the Mvera cattle strain, because both drew trypanosomes from 
the same two sources ; but we cannot determine whether the difference in the ox 
is due to difference of the host, or to the introduction of two more sources. Simi- 
larly the divergence between the trypanosomes from rat and from other animals for 
the wild-game strain may be due to using one rat and therefore one source, and not 
the many sources of the other animals, or it may really be due to the differentiation 
of the host. In the same way the difference between the two hartebeeste may be 
due to individuality in the same .species, or to infection from different strains. 
{d) To some slight extent we may appreciate the effect of individuality by 
comparing the two rats 512 and 513 in the case of the single source, the Mzimba 
strain J. 
The frequencies are as follows : 
Microns. 
Mzimba Strain 
16 
17 
18 
19 
"21 
22 
2S 
2^ 
25 
26 
27 
28 
29 
30 
Totals 
Rat 512 
5 
17 
37 
36 
26 
25 
18 
22 
14 
9 
11 
9 
7 
1 
1 
2 
240 
Rat 513 
2 
9 
24 
54 
43 
30 
28 
20 
17 
8 
10 
5 
6 
2 
1 
1 
260 
The numbers are not as large as we should like ; but they give 
= 17-89, P = -:3306. 
* R. S. Proc. Vol. 87, B, pp. 2 and 15. 
t R. S. Proc. Vol. 87, B, pp. 6 and 8 compared with 5. Kat from p. 8. 
+ R. S. Proc. Vol. 87, B, pp. 29 and 31. 
Biometrika x 14 
