372 A HISTORY OF BIRDS 



with the more normal avian apparatus : and this is a very im- 

 portant consideration where small living prey has to be captured 

 on the ground during the hours of twilight. 



Embryology, so far, has thrown but little light on this subject. 

 In an embryo Long-eared Owl I found a stage exactly compar- 

 able to that which obtains in the living Scops Owl, but no trace 

 of the operculum was then visible. The history of this region 

 has yet to be traced. 



Another curious feature is to be seen in the form of the 

 Squamosal in the nestling skulls of the Tawny and Burrowing 

 Owls. This bone has pursued very different lines of evolution 

 which, moreover, do not seem to be explicable as the result of 

 natural selection, but rather to belong to the category of char- 

 acter referred to elsewhere in these pages ; to wit, characters due 

 to variation in the germ-plasm, which are free to pursue their 

 course unless, and until, checked by natural selection ; that is, 

 until they tend to hamper well-being. This point has been 

 more fully described in my Memoirs in the Transactions of the 

 Linnean Society, 1898. 



From the problem of shape we have already passed to that . 

 deeper one of structure, and this we will now pursue further, 

 tracing out those peculiar modifications of the type which we 

 may justly regard as the direct response to the demands of the 

 environment, or to the particular needs of the animal, or, in 

 other words, as adaptations. 



The selection of these instances of adaptation must in a 

 sense be arbitrary, inasmuch as the whole organism is but the 

 sum of innumerable adaptations. On the theory of evolution 

 birds are the descendants of reptiles. So the gradual shedding 

 of the reptilian characters, or to put it another way, the gradual 

 modification and transformation of the reptilian characters, and 

 their transmutation into avian, are so many steps in adaptation 

 towards the new demands of the new environment. Thus, 

 feathers have been derived from scales by adaptation ; the trans- 

 formation of the ambulatory fore-limb into a wing is the result 

 of adaptation. But we propose to use this word adaptation in its 

 more general sense, to indicate more or less striking departures 

 from the type. Thus commencing with what we may call the 

 schematic bird, we have an animal clothed with feathers, hav- 

 ing the fore-limbs modified to iorm organs adapted for aerial 



