482 Letters, Announcements, ^c. 



of an alteration from a conspicuous colour to one resembling 

 the hue of the surrounding 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 varie- 

 ties 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 pecu- 

 liarity 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. If it be objected that we see many varieties which 

 do not become hereditary, we may reply, that these varieties, 

 having experienced changes not advantageous to their means 

 of existence, may from that very cause become extinct. Still 

 there are many which continue, as the Pied Raven of the 

 Faroe Islands and the Tailless Manx Cat. 



" To apply the theory to the case of the Sahara. If the 

 Algerian Desert were colonized by a few pairs of Crested 

 Larks, — putting aside the ascertained fact of the tendency of 

 an arid hot climate to bleach all dark colours, — avc know 

 that the probability is, that one or two pairs would be likely 

 to be of a darker complexion than the others. These, and 

 such of their offspring as most resembled them, would become 

 more liable to capture by their natural enemies, Hawks and 

 carnivorous beasts. The lighter-coloured ones would enjoy 

 more or less immunity from such attacks. Let this state of 

 things continue for a few hundred years, and the dark-coloured 

 individuals would be exterminated, the light-coloured remain 

 and inhabit the land. This process, aided by the above- 

 mentioned tendency of the climate to blanch the coloration 

 still more, would in a few centuries produce the Galerida 

 abyssinica as the typical form. And it must be noted, that 

 between it and the European G. cristata there is no distinc- 

 tion but that of colour. 



"■ But when we turn to Galerida isabellina, G. arenicola, 



