454 A HISTORY OF BIRDS 



There certainly seems to be some show of evidence that 

 habit precedes structure. The case of the Diving Petrel 

 ( Urinatrix) will serve as an illustration. Though superficially 

 this bird so closely resembles the little Auk {Alca alle), yet, in its 

 skeleton, it has undergone but little modification of the normal 

 Petrel type. The Pluvialine Alcidae, e.g., Guillemot and Auk, 

 have apparently, from a longer course of moulding, greatly 

 modified the skeleton of the trunk, and to a less extent of the 

 legs. The marine Divers {Colymbidce) and lacustrine Grebes 

 have from a yet longer sojourn in their respective environments 

 suffered still further changes ; but all the types in question 

 have developed the same general features, as of course was to 

 be expected. 



Before closing this chapter a brief reference must be made 

 to the opposite side of this picture, wherein is presented a 

 number of equally remarkable phenomena illustrating that in- 

 teresting phase of evolution known as parallel development. 



Parallel development differs from convergent development 

 in this, that whereas in the latter, forms not even remotely re- 

 lated have come to assume a superficially identical appearance, 

 in the former that resemblance is due to a common descent, 

 albeit remote, before heterogeneity had well begun. Often this 

 parallelism is worked out in regions of the earth far distant 

 one from another, and always before the several characteristics 

 which distinguish the living descendants of this primitive 

 stock had assumed their final trend ; that is to say, while 

 these characteristics were yet in a nascent condition so that 

 the differences which obtain between what we now call the 

 parallel forms are to be regarded as so many " expression 

 points " of environmental influence — of selection — and of the 

 struggle between the several parts of the organisms involved, 

 as a consequence of their early separation and long isolation. 



It is an open question whether the New and Old World 

 Vultures are to be regarded as convergent or parallel types. 

 The Old World Vultures are unquestionably Accipitrine ; but 

 it is not so certain that their doubles of the New World are 

 descendants of nascent Accipitres, or are offshoots from the 

 Gruiform stem, or, as the late Professor Garrod held, of the 

 Ciconiform stem. Forbes hinted at a descent from the Petrels, 

 and in many parts, especially the cranial characters, there are 

 striking resemblances thereto. 



