A Survey of the Scorpion Fauna of South Africa. 
95 
tion to his paper on that geuus. Again, the stability of certain specific types 
is amply proved by their wide distribution. Opisthophthahnus karrooensis 
is a very common species in the Karroo. Specimens of this species from 
Jansenville seem to be identical (except in size) with material from Williston 
and Carnarvon, more than two hundred miles away : these sluggish creatures 
distribute slowly, and probably hundreds or even thousands of generations 
separate the Jansenville and Williston specimens from their common 
ancestor. 
The species JJroplectes triangulifer Thorell has a very much wider range, 
and though variable within narrow limits is clearly a natural unit. 
According to the evolutionists, discontinuity within a genus may be 
<;onnected with two distinct types of variation : either, characters which 
behave as indivisible units in inheritance may be present or absent ; or, the 
variations of a character may take the form of a gradational series from one 
extreme to the other, discontinuity arising by the disappearance of one or 
more members of the series. In reality the two types of variation cannot be 
very sharply distinguished, for all variation proceeds by jumps which only 
differ in magnitude ; there is an important difference, however, in the process 
of evolution, for whereas in the former case discontinuity may arise solely as 
a result of internal impulses, in the latter case some outside factor such as 
natural selection or isolation is required to complete the process. From an 
examination of the few facts that are available to us, the broader facts of 
distribution and of variation, I am inclined to suspect that both methods 
have contributed to the diversity among scorpions. When we see two 
closely allied species living together in precisely the same environment, it is 
difficult to understand how natural selection or any other external factor can 
have brought about a splitting of the original common stock. Such species 
are not generally so closely related in structure that they can be regarded 
with certainty as the descendants of an immediate common ancestor, yet 
sometimes {cp. Opisthophthalmus glahrifrons and 0. latimanus pugnax, which 
both occur in the vicinity of Pretoria) the species concerned are without 
doubt more nearly related to each other than to any other known species. 
On the other hand, the occurrence of numerous local forms of a species 
seems more readily explained as an indirect consequence of isolation or some 
environmental factor acting on the several portions of a variable stock. 
The suggestion that the larger units, commonly called species, which 
coexist in the same environment, may arise by mutation processes, whilst 
the minor units known as " little species," or geographical or topographical 
varieties, arise through a process of continuous variation, is one which can 
best be tested through a critical examination of the constancy or otherwise 
of the characters concerned. The value of the characters employed by 
systematists in the discrimination of species of the genus Opisthophthalmus 
