-264 COLOURATION OF REPTILES AND AMPHIBIANS. 



assimilate closely with those of the stones of the immediate 

 neighbourhood. It never climbs trees, but the male assumes 

 most brilliant colours during the breeding season, more especially 

 on the ventral surface. The ventral surface of the adult male is 

 •ornamented with a bright ultra-marine blue, extending from chin 

 to vent : this is very changeal)le, and in a captured specimen may 

 give place to a dull mud colour in a few minutes. There is also 

 a deep lemon colour bordered with pin^ple and red in the inguinal 

 region : this remains permanent during the breeding season. 



Another lizard, Zonurus polysonus, inhabits the same kopjes, 

 and the same individual may occupy a particular rock crevice for 

 many months, never venturing far therefrom. It may also be 

 found on small ridges, but only where the vicinity offers rocks or 

 stones with suitable hiding-places. The dorsal colouration again 

 is entirely protective, but unlike the Agamas, is not capable of 

 modification — at least, not during a short space of time — and 

 experiments with varying light and temperature have prodviced no 

 responsive change in the dorsal colovn-s. This fact may be con- 

 nected with the extremely sedentary habit of the animal : amongst 

 the black rocks which it frequents, the shining black of its coat 

 harmonizes perfectly. 



The ventral colours of this creatvu^e are very different. The 

 whole ventral surface of Zonnnts polyzonus from chin to vent is 

 brilliant red, and the rich red colours are possessed by adults 

 of both sexes throughout the year. The reptile may often be 

 seen perched on the top of a rock, sitting as erect as the length of 

 his forelimbs will permit, and thus displaying to best advantage 

 his flaming vest. It is noteworthv that on the approach of danger 

 iDoth the Agama and Zonurus hide their bright ventral colours by 

 pressing the body against the rock or other object on which the 

 animal may be resting. This sufificiently indicates that the bright 

 ventral colours cannot have a warning significance. 



The young of Zononis polysoiiits clo not develop a red col- 

 ouration on the ventral surface until the second or third rear. 



Some light is thrown on the efficiency of the life-preserving 

 mechanisms possessed by animals through a comparison of their 

 birthrates, the total population of a species being assumed as 

 constant. 



From examination of a large mmiber of females I have found 

 that Zonnriis polysomis produces only from one to four young, 

 generally two, at a time, while the Agamas lay from fourteen to 

 seventeen eggs at a time : in the latter case, however, there is no 

 doubt some mortality amongst the eggs. 



In the " Origin of Species,'" page 83, Darwin says : 



The real importance of a large number of eggs or young is to make uy> 

 for much destruction at some period of life ; and this period in the great 

 majority of cases is an early one. 



Regarding this as a truism, it Avould appear at first sight sur- 

 prising that Zonurus, which does not possess the power of colour 

 change, has less mortality than the Agama and certain of the 

 skinks which can thus adapt themselves ; yet the fact is that 



