'92 Transactions of the Royal Society of South Africa. 
If the investigation be limited to such a genus as Hadogenes, their views 
may be seen to be confirmed. In that genus there are numerous local forms, 
the extremes of which differ greatly from each other, though the form of any 
one locality is more or less constant in structure but of limited range of 
distribution. These forms are conveniently arranged into a small number 
of groups called species, but it may perhaps be found that the lines have 
been drawn somewhat arbitrarily between the groups, and certainly the 
differences between them are only of the same kind as though greater in 
magnitude than those between the several forms of the same group. In the 
genus Hadogenes, so far as I can ascertain, no two species or varieties are 
ever found living together in precisely the same locality, and one might 
therefore be inclined to suspect that all the various forms are only geo- 
graphical races of one species. Hadogenes luhitei, as found near G-rahamstown, 
is quite different from H. gracilis occurring near Pretoria, but the two are 
linked together through the Transkei and Cradock varieties, which incline 
definitely towards whitei, being now included therewith in the species 
H. trichiurus Grerv., and through the O'okiep variety, which on the whole 
approaches more closely to gracilis ; we seem compelled to anticipate that a 
complete series of gradational forms must be found to exist between the two 
extremes. 
I do not feel satisfied, however, that the whole genus is connected up in 
this way. Although, as a general rule, those forms which are geographically 
adjacent are also more closely related to each other in structure than those 
which are widely separated in space, yet it sometimes happens that forms 
which differ widely from each other in structure are to be found living in 
close proximity though perhaps not actually together. This is the case in 
the Pretoria district, where two well-defined species occur, viz. H. gunningi 
Purcell and H. gracilis Hewitt. They occupy distinct mountain ranges in 
that district, and, according to Mr. G. van Dam, who has extensively col- 
lected these species, they do not occur together. So far as we can judge, 
these two species are not associated with different environmental conditions, 
and it would seem probable that the only external factor contributing to the 
maintenance of structural difference is isolation. 
According to Prof. Bateson, " the only definable unit in classification is 
the homozygous form which breeds true " ; and again he says, " these little 
species, finely cut, true-breeding, and innumerable mongrels between them, 
are what one finds on examining any so-called variable type." This sugges- 
tion no doubt contains an element of truth, but I think the importance of 
the " little species " is greatly exaggerated by Bateson. If a large series 
of scorpions belonging to a single species and collected within the same 
limited area be carefully examined, we shall find a high degree of constancy 
in certain characters but a fairly wide range of variation in others. I have 
i 
