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. 



