294 
GEOFFREY SMITH. 
detection as those Avhich can be shown ( 10 ) to invalidate the 
apparently well-established results on the thumb of the frog-. 
It is clear that if the theory developed in this paper is 
well founded, we should not expect that injections of ovarian 
or testicular substances, or of substances derived from the 
Sacculina roots, would have any effect in calling forth the 
development of sexual characters. There is no reason for 
supposing that an emulsion or extract of these organs would 
contain the fatty side-chains in a condition capable of 
assimilation, and of setting on foot the complicated nexus 
of metabolic processes which results in the progressive 
alteration of the liver and of the blood. 
We have seen that the whole effect of Sacculina on its 
hosts is consistently explained by the theory vve have adopted, 
a theory which may be described as metabolic stimula- 
tion. 
Let us see how far short the Hormone theory falls in a 
similar attempt. Since the infected males develop female 
secondary sexual characters in the absence of an ovary, the 
Hormone theory offers us two alternatives : either the Saccu- 
lina roots secrete a hormone which acts on the crab, or else 
the mere suppression of the testis liberates a hormone from 
the crab which brings about the secondary sexual changes. 
To take the last alternative first, it is quite possible that 
the mere suppression of the testis might call forth the 
development of female secondary characters, but it is diffi- 
cult to see how the mere suppression of the testis should 
make the crab subsequently develop an ovary. But even if 
we grant this highly improbable result, why should the sup- 
pression of the ovary in the young female crab influence the 
latter to assume prematurely adult female secondary cha- 
racters ? The explanation in fact falls to pieces when we 
try to apply it to the whole of the phenomena. 
It is the same with the other alternative, viz. that the 
Sacculina roots secrete the hormone. This would explain 
why the female secondary sexual characters should be 
developed, but how could it explain the subsequent forma- 
