302 GARY N. CALKINS 
The relative vitality of the series now living (March 10, 1920, 
series W, X, XZo, Y, Z, b, c, d, e, f, g, h, 1,7, and k) cannot be 
accurately computed, although a few are far enough along to 
furnish some evidence. The problem of in-breeding is compli- 
cated by that of age of parents at time of conjugation, for no 
chain of filial generations is free from some weak link due to 
this cause. The oldest series in number of filial generations is 
k in the ninth filial generation. All of its ancestors up to the 
a series had high relative vitality, but the a series, because of 
the old age of its parents, had next to the lowest vitality of all 
series yet cultivated. Efforts are now being made to build up 
the vitality of this strain through the b, e, and k series. ‘The 
Diagram 2 
next oldest strains run through eight filial generations ending 
in the hf and 7 series now living, and the weakest links will prob- 
ably be the XZ. series which came from the 270th generation 
of the V series, and the U2 and ¢ series which came from parents 
in their 208th generation. The three oldest strains thus have 
the same early, or common, ancestry as far as the L series. 
If continued in-breeding has had any deteriorating effect on 
the vitality of offspring, we should expect a lower, vitality in the 
later filial generations. Table 3 gives the relative vitality of 
all the series in question; the A series stands at 90.9 per cent, the 
first filial generation, © series, at 97.7 per cent, the second filial 
generation of F series, at 94.1 per cent, the third filial generation 
or I series, at 89 per cent, the fourth generation, or L series falls 
to 74.5 per cent, and this might be interpreted as an effect of 
in-breeding were it not for its progeny of the fifth filial generation, 
