May, 1891.] THE BRITISH NATURALIST. 



89 



Vertebrata. 



"WHAT IS A BIRD?" 



BY LINN^US GREENING. 



( Continued from page 27.) 



Having briefly considered the most important structural and 

 functional resemblances between birds and reptiles, we may now 

 proceed to show how birds have descended from reptiles, to try to 

 realize how the feathered flying form has arisen from the scaled creep- 

 ing one. The facts are enough to warrant us in asserting that this 

 has happened, but how we can only conjecture. Having found in the 

 Oolite strata fossils of true feathered birds, A vdwopteryx mo cruv a, ^hich. 

 however, differ from existing birds in the structure of the tail, we are 

 forced to assume that there must have been a long line of ancestors 

 between these and the first small, four-legged, active, insectivorous, 

 scaly tree lizard from which they were descended. Some of the des- 

 cendants of this early tree lizard, which were of a more active 

 disposition than their ancestor, would have a higher temperature, 

 which would be accompanied by some modifications of structure and 

 of the size and shape of the scales. It is clear that any tendency of 

 the scaly coverings to become downy, would preserve, nay, would 

 increase the warmth of the body, accompanied by still further modi- 

 fications of the scales. This hypothesis is confirmed by a study of 

 the development of a feather which in its earlier stages cannot be 

 distinguished from a scale. In other words, up to a certain point of 

 its growth a feather is exactly like a scale, but it subsequently 

 becomes broken up and subdivided to an extent that renders it one of 

 the most wonderful modifications which occur in the animal kingdom. 

 Descendants of the first down covered reptiles would vary from the 

 parental type as it had varied from its ancestral form. These varieties 

 would be cumulative and would be accompanied, cumulatively, by 

 increased activity, increased temperature, and increased modification 

 of the down, with the result that in time there would be active, 

 insectivorous tree-lizards, which, instead of being covered with scales 

 would possess an epidermic covering, consisting at least in part, 

 of simple feathers. It will be observed that I say simple feathers, 



