304 GARY N. CALKINS 
On the whole, there is no evidence that continued in-breeding 
and under the same cultural conditions for more than two years 
had had any deleterious effect on vitality of the protoplasm 
under observation. The cyclical exhaustion of vitality is con- 
tinuously offset by renewal through conjugation, and the extent 
of this renewal depends upon the age at which the parents con- 
jugate. The optimum of vitality is apparently assured by con- 
jugation of parents during their period of youth, and the relative 
strength of this optimum varies with the congenital vitality of 
the parental protoplasm. 
SUMMARY 
From the results described in the second of these Studies 
(Calkins, Uroleptus mobilis. II. J. Exp. Zodl., vol. 29, no. 2) 
and on a priori grounds, we might expect that, apart from slight 
variations of a casual nature, all series derived from conjugation 
would have practically the same vitality since all are representa- 
tives of the same protoplasm and all began their life- cycles as 
ex-conjugants from closely related parents. 
This a priori expectation has not been realized, however, and 
striking variations in vitality between the seventeen series of 
Uroleptus mobilis which have completed their life-cycles and 
variations which cannot be interpreted as casual have been 
obtained. Some series have run as high as 97.7 per cent, 95 
per cent, and 94.1 per cent of an ideal perfect vitality (series C, 
P, and F), while others have been as low as 5.4 per cent, 44 per 
cent, and 48.1 per cent (series Q, a and R). 
It has been shown previously that all series of Uroleptus mobilis 
begin life after conjugation with an initial optimum vitality which 
gradually and inevitably diminishes until the protoplasm finally 
dies with unmistakable evidence of the exhaustion of metabolie 
activities. In the present paper it is shown that the striking 
variations in vitality of different series are due to the age, and 
therefore to the relative vitality of the protoplasm of the parent 
cells at the time of conjugation. All series with extremely low 
vitality agree in coming from parents which were in the period 
of old age at the time of conjugation, while all series with 
