22 
THE VOYAGE OF H.M.S. CHALLENGED. 
one of these I will point out myself. Those species which are unisexual and have very 
small and rudimentary males, which, therefore, according to the sketch given above, are 
the youngest of the hypothetical course in which the different stages of sexual differenti- 
ation have developed, are at the same time those which closely resemble the species from 
the oldest geological strata from which species of Sccdpellum are known. But I think this 
objection is weakened by admitting that the sexual differentiation in the genus Sccdpellum 
was already achieved at the period from which these fossil remains date. The somewhat 
aberrant shape of Sccdpellum balanoides — the species in the supposed original condition 
of true hermaphroditism — is also difficult to explain at first sight. We might have 
expected to observe the original condition (hermaphroditism without complemental males) 
in a species as closely resembling Pollicipes as possible, as, for example, in Sccdpellum 
villosum, Leach, sp. ; or Sccdpellum trispinosum, Hoek. 1 The condition of the genital 
apparatus and the external shape of the valves (the whole capitulum), however, are two 
factors which need not necessarily stand in so very close a relation to each other. So it 
may be easily imagined that the original condition of the sexual apparatus is left in a 
form in which the external shape of the capitulum has been altered, and, on the other hand, 
there is no reason why the sexual relations of a form should not become altered without 
the external shape undergoing considerable changes at the same time. 
When, however, all these considerations are weighed I do not believe that there are 
trustworthy grounds for doubting the exactness of the hypothesis that in the genus 
Sccdpellum the hermaphrodite condition is the original, and the unisexual the secondary 
stage in the development. 
1 I (lid not observe the male of Scalpellum trispinosum. I suppose that this species is furnished with a comple- 
mental male with capitulum and peduncle from its resemblance to the species Scalpellum villosum. I did not study its 
genital apparatus. I can only say that it is furnished with a well-developed penis. 
