Chapter XI 



THE OCULAR FUNDUS OF BIRDS IN ITS RELATION TO 



A CLASSIFICATION OF AVES 



The writer is well aware of the controversies 

 so easily provoked in that zoological storm 

 centre, the systematic classification of Birds, 

 but he believes that as the fundus appear- 

 ances in wild species are probably invariable 

 and that, as the evidence so far produced 

 shows, each species exhibits a background 

 picture distinct in one or more particulars 

 from every other species, it is possible to 

 identify many of them by the use of the oph- 

 thalmoscope alone; in other words, in the 

 classification of Birds the fundus details, 

 including the varied coloration of the eye- 

 ground, should be considered quite as much 

 as the histology of any organ or division of 

 the body. 



The extent to which these taxonomic indi- 

 cations correspond to the orders of any par- 

 ticular classification cannot, in the absence of 

 a complete or nearly complete investigation, 

 now be determined. In the opinion of the 

 writer, however, there are certain facts that 

 have arisen in the course of this preliminary 

 research that are worth discussing for the light 

 they throw upon and the aid they may give 

 to a systematic arrangement of avian species. 



That the eyegrounds of the Ratite orders 

 present peculiarities that distinguish them 

 from the Carinatoe is evident at a glance. On 

 the other hand there are the usual differences 

 between the acarinate species. The colora- 

 tion of the fundi and the characters of the 

 pectens in Rhea (Plate II) and Struthio 

 (Plate I) bear a close resemblance. The 

 ocular fundus of the Cassowary (Plate III) 

 is more closely allied to that of the Kiwi than 

 to the Nandu. The ocular background of the 

 Tinamou is decidedly acarinate and this fact, 



with other taxonomic indications, justify, 

 from the ophthalmoscopic viewpoint, its in- 

 clusion in that subclass. 



The Apteryx is so decidedly a nocturnal 

 animal - — especially in the use of its eyes for 

 distinct vision both in the distance and near 

 — that one is not surprised to see so much 

 yellow in its fundus coloration. There are 

 so few exceptions to the rule of the yellow- 

 colored fundus in Night Birds that one may 

 confidently assert that the amount of this 

 color in avian (one might say in all verte- 

 brate) fundi is in direct ratio to the propor- 

 tionate use a given species makes of its visual 

 powers after sunset. When a bird and his 

 ancestors (in the evolutionary sense) have 

 used their eyes for distinct visualization 

 largely or exclusively at night the fundus tint 

 is nearly always yellow or orange. 



Applying this rule to carinate Night Birds 

 we find many consistent and marked ex- 

 amples of it. For instance, all the Strigi- 

 formes so far examined have a preponderance 

 of yellow in their eyegrounds. Without 

 carrying this assertion too far we note that 

 in semi-nocturnal Owls, like Strix flammea 

 (Plate XXXVIII), the red tints overpower 

 the yellow and a shade of orange results. 



Still more suggestive are the fundal con- 

 ditions seen in the Owl Parrot, Stringops 

 habroptilus (Plate XL). Compare the fun- 

 dus, especially its yellow color and the posi- 

 tion of the (temporal) area centralis, first 

 with that of any Owl and then with Psitticus 

 amazona; undoubtedly, so far as the eyes are 

 concerned, they belong to the former. 



Another observation of avian fundi seems 

 to show that an admixture of yellow (in the 



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