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PACIFIC SCIENCE, VoL XIII, April, 1959 
relation of geographical speciation in leaf- 
mould animals with major events in past 
geological history, including glaciation. Ac- 
cording to Forster (1954), the opilionid fauna 
of Stewart Island "suggests a series of inva- 
sions and reinvasions from the mainland fol- 
lowing periods of isolation of sufficient time 
length to permit physiological isolation to be 
achieved." Further, "the distribution of the 
fauna of the West Coast of the South Island 
is strongly suggestive of bio-glacial control, 
paralleling as it does that of the Paryphanta 
snails for which Willett (1950) postulated a 
similar origin.” 
The formation of ecological islands, to 
which leafmould is particularly susceptible by 
the presence of such faunal barriers as rivers, 
mountain ranges, and valleys, also provides a 
marked stimulus to speciation. This has been 
noted for other leafmould animals. Speaking 
of the land molluscs of Fiordland, Dell 
(1955) comments ". . . such isolated pockets 
would form ideal localities in which speciation 
could proceed. The comparatively high num- 
ber of forms of such a genus as Ptychodon 
could well be due to such a situation, coupled 
with subsequent invasions." 
The mechanism of speciation is perhaps 
less obvious but I would suggest that the 
most probable is speciation by neotony— the 
retention of juvenile or embryonic characters 
in the adult. 
In most species of Orchestia, including all 
of the supralittoral ones, the second gnatho- 
pod is large and subchelate in the male, and 
small and feebly chelate in the female. In 
Talitrus the male second gnathopod is small 
and feebly chelate, superficially indistinguish- 
able from that of the female. Since the greatly 
developed male gnathopod in Orchestia is 
known to be used for grasping and even 
"carrying" the female in coitu, there is a strong 
likelihood that the large size of the male 
gnathopod is unnecessary in Talitrus because 
of changed sexual habits. 
The terrestrial species of Orchestia, unlike 
the supralittoral ones, show gradations in size 
and type of male gnathopod from the typical 
Orchestia form to a form not far removed from 
the typical Talitrus gnathopod. These "inter- 
mediate” species show striking resemblances 
to the various stages in the development of 
the male second gnathopod in supralittoral 
species of Talitridae. A typical series is shown 
in Figures 2 and 3, where various adult male 
second gnathopods of "intermediate" forms 
are contrasted with growth stages of Talor- 
chestia hottae (figured by Chilton, 1921, as T. 
martensii). The suspicion that these "inter- 
mediate" forms are no more than subadult is, 
I believe, unwarranted. All other evidence 
from morphology and distribution points to 
the animals being genuinely mature. 
The other striking morphological feature 
of the terrestrial species is the trend to reduc- 
tion of pleopods from the normal supralit- 
toral and marine condition, with three pairs 
of fully-developed biramous pleopods, to the 
extreme in 0. patersoni, a terrestrial spe- 
cies with three pairs of vestigial triangular 
stumps. The possibility that this trend is also 
an effect of species formation by neotony has 
been considered, but limited examination of 
supralittoral juveniles showed fully formed 
pleopods at an extremely early stage in de- 
velopment. While it is possible that pleopod 
formation passes through a bud stage similar 
to the vestigial stump, it would have to occur 
at a very early stage and would probably be 
revealed only by study of the late embryo. 
Nevertheless, the diagrammatic representa- 
tion of pleopod structure in Figure 3 indicates 
that reduction of pleopods and reduction in 
size of male gnathopods occur more or less 
in phase and are not inconsistent with the 
process of neotony. 
MORPHOLOGICAL AND 
PHYSIOLOGICAL ADAPTATIONS 
Work on the systematics of the terrestrial 
amphipods has revealed some interesting 
tendencies in adaptation. There are no mor- 
phological changes of any significance in the 
mouth parts, but in view of the similarity of 
