394 
SAVAGE SUDAN 
those various creatures have been catalogued and classified, and 
it may be unwise, even cursorily, to allude to things—however 
striking—of which one has no technical knowledge whatever. 
Thus, for example, alongshore one comes across colonies of 
weird land-crabs, which build regular villages of pyramidal 
watch-towers, upon the apex of which each crustacean owner 
basks in the sunshine; but each alert, on the approach of 
danger, to slither down sidelong to the refuge of his burrow 
just below. 
To me the most striking observation recorded related to 
the big desert-larks—Certhilaudas. These I have already de¬ 
scribed (pp. 28 and 357), yet am bound to add this further note. 
A Colony of Watch-tower Crabs. 
Here, on the coast, the local race was conspicuously distinct 
in colour-plan from my recollection 1 of those of the interior 
deserts, being of a clear pale grey (which Is a cold colour), as 
against dun or drab (which is warm). 
Apparently, in this instance, Nature had admitted a slight 
slip!, That is, she had in practice failed to fulfil her principle. 
For it is.obvious that these dove-grey Certhilaudas of the littoral 
would better assimilate with the blue-grey shingle of the desert 
than do their sandier colleagues of that ilk—and vice versa. 
The two ought to change places! 
Here, on the coast, the Certhilaudas were preparing to nest 
1 A recollection corroborated a few days later on the deserts beyond 
Jebel Surgham, though hardly so strongly as I had anticipated. On the 
true desert, the Certhilaudas were distinctly sandier ,, yet showed a faint 
cast of grey on mantle, nape, and crown. 
