ECOLOGY AND EVOLUTION 183 



5. There are many similar cases of dimorphic forms which 

 must have arisen by the spreading of colour varieties in the 

 population, but which apparently cannot have been encouraged 

 either by natural or sexual selection. Another case similar to 

 that of the arctic fox, is the antelope called the white-eared 

 cob {Adenota leucotis)^ which inhabits the steppe country of 

 the upper Sudan.^^^ In this antelope there are two colour 

 phases, one of which is light or tawny in colour and more or 

 less matches its surroundings, while the other is dark or almost 

 black. The interesting thing is that this colour dimorphism 

 exists only in the male, the females being all light- coloured. 

 The light individuals match their surroundings, the black do 

 not. Taking the whole range of the species, there is an area 

 in the middle of the range with black and light phases living 

 together, and an outer zone with the light phase only. 

 Examination of the horns showed that the differences in colour 

 in the males were not due to age differences, as is often the 

 case with such animals. 



Here again, what at first sight seems to be an admirable 

 adaptation in colour, turns out to be no better off than its com- 

 panion phase which does not match its surroundings at all. 

 Even if we assume that the colours are correlated with some 

 other adaptation, the difficulty remains. There are, of course, 

 a vast number of cases in which effective colour adaptation 

 almost certainly exists {e.g. in the ptarmigan and in many 

 insects), but in these cases there are never important dimorphic 

 phases. But the fact that the adaptation exists in a number of 

 cases does not in any way affect the fact that in certain other 

 cases it does not exist at all. 



6. It is rather interesting to find how emphatically nearly 

 all naturalists who have had wide experience of wild mammals 

 reject the idea of colour adaptation in these animals. Dug- 

 more ^^^ says : " The whole theory of protective coloration 

 in the larger animals may be open to argument, but from my 

 own observations in the field I am firmly convinced that 

 practically speaking there is no such thing," while Roosevelt 86b 

 says : "In South America concealing coloration plays no 

 more part in the lives of the adult deer, the tamandua, the 



