CANON TRISTRAM'S SUPPORT. 93 



In all these birds we trace gradual modifications of coloration 

 and of anatomical structure, deflecting by very gentle grada- 

 tions from the ordinary type ; but when we take the extremes, 

 presenting most marked differences." 



These differences, he concludes 



"have a very direct bearing on the ease or difficulty with 

 which the animal contrives to maintain its existence." 



He then points out, upon the uniform surface of 

 the desert it is absolutely necessary that animals shall 

 be protected by their colour : 



"Hence, without exception, the upper plumage of every 

 bird, whether Lark, Chat, Sylvian, or Sandgrouse, and also the 

 fur of all the small mammals, and the skin of all the Snakes 

 and Lizards, is of one uniform isabelline or sand colour. It is 

 very possible that some further purpose may be served by 

 the prevailing colours, but this appears of itself a sufficient 

 explanation. There are individual varieties in depth of hue 

 among all creatures. In the struggle for life which we know 

 to be going on among all species, a very slight change for the 

 better, such as improved means of escaping from its natural 

 enemies (which would be the effect of an alteration from a 

 conspicuous colour to one resembling the hue of the surround- 

 ing objects), would give the variety that possessed it a decided 

 advantage over the typical or other forms of the species. 

 Now in all creatures, from Man downwards, we find a 

 tendency to transmit individual varieties or peculiarities to 

 the descendants. A peculiarity either of colour or form soon 

 becomes hereditary when there are no counteracting causes, 

 either from change of climate or admixture of other blood. 

 Suppose this transmitted peculiarity to continue for some 

 generations, especially when manifest advantages arise from 

 its possession, and the variety becomes not only a race, with its 

 variations still more strongly imprinted upon it, but it becomes 

 the typical form of that country." 



Canon Tristram then points out the manner in 

 which he imagines that one of the crested larks of 



