A Survey of the Scorpion Fauna of South Africa. 97' 
same spot. The two species 0. austerus and 0. harrooensis, which both 
occur at Victoria West, are also mainly separated from each other by this 
character, though certainly other but minor points of distinction do exist. 
In these several pairs of species the character concerned shows the con- 
stancy of an absolute unit, for no indication of intermediates occurs. It 
would appear therefore that the same process has taken place quite inde- 
pendently in three different areas operating on three different stocks, and 
resulting in each case in the formation of a pair of similarly contrasting 
species. That process might conceivably be a different one in each case,, 
but if I am right in believing that the individuals of a pair do normally 
live side by side, it would seem that the simplest explanation of the facts 
is to be found in the mutation hypothesis. Still, we may safely say that 
the various species of Opisthophthalmus are not to be regarded as so many 
rigidly separated units, for the existence of such mathematical units must 
depend upon the occurrence of absolute unit characters, which alone can 
be used in the discrimination of species as thus understood. It is probably 
quite true that 0. ijallicli'})es Koch, and 0. pering^ieyi Purcell, are rigidly 
separated from each other through the character previously mentioned in 
the locality where these species occur together ; but, unless the characters 
which mark off these two species from the rest of the genus are also quite 
sharp, there is a possibility that structurally the two species may be con- 
nected up, though very indirectly, through other species. In view of the 
continuous variability of most of the characters employed in the distinction 
of species amongst our scorpions, it seems probable that discontinuity arises 
most frequently through the elimination of intermediate forms rather than 
by mutation processes. Perhaps it is more apparent than real, and the 
gaps now existing may be filled up by the material of future collectors. 
Evolution amongst scorpions has not proceeded altogether independently 
of the environment. Some of the species are obviously confined to particular 
environmental regions. This is the case, for example, with Opisthophthalmus 
Jcarrooensis, which is specially associated with the karroid regions of the 
Cape, and Dr. Purcell believes that a narrow strip of country along the 
Western Cape Coast, characterised by winter rains, is the peculiar habitat 
of other species. It is significant that in each of the larger genera — 
Opisthophthalmus, Parabuthus, and Uroplectes — we find one representative 
whose distribution area is the southern coastal strip of the Cape Province. 
In Opisthophthalmus we may roughly divide the genus into a Western 
section characterised by the presence of a V-shaped groove on the frontal 
area and an Eastern section which has no such groove. Now in a southerly 
direction the Western section extends as far eastwards as Port Elizabeth, 
and this same locality is the south-western limit of the Eastern group of 
forms included under the name of 0. latimanus. In the genus Uroplectes,. 
