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THE AMERICAN NATURALIST. [Vol. XXXVIII. 



forms from which not only have the ostriches arisen, but both 

 modern birds and reptiles have been derived. 



In the classification of existing birds, again, we have recourse 

 to their anatomical structure or morphology, and incidentally their 

 physiology to assist us, the first of these being a very powerful 

 aid. By morphology is meant the science of the outer form and 

 internal structure, and to be of practical value in taxonomy it 

 must be made thoroughly comparative. As far as possible we 

 also study the morphogenesis of birds or the genesis of form in 

 their case through evolution. Of great assistance is the knowl- 

 edge we derive from our researches into the comparative ptilosis 

 and ]:>terylography of the group and all that pertains to these 

 related sciences. In our efforts, too, to seek out the true rela- 

 tionships of birds in order to arrive at a natural classification we 

 must take into consideration, in the broadest possible sense, 

 their embryology ; their oology and nidiology ; their habits and 

 comparative longevity, and even their various notes and songs 

 must be given due weight. Their geographical distribution, a 

 very important factor to be studied in their taxonomy, is to be 

 considered not only from the viewpoint of the present distribu- 

 tion of the species of the class over the earth's surface, but like- 

 wise a comprehension, as far as possible, of the question as to 

 how that distribution came about. The data for the latter, in 

 the case of birds, are extremely meagre, as it takes into consid- 

 eration the migration and original habitats of various forms of 

 the class during geologic times, and as but very few fossil 

 remains of birds have been discovered in any part of the world, 

 there has been in consequence but very little light thrown upon 

 this latter side i)f the question. So far as it goes, however, it 

 has its value, as has also, to some extent, a consideration of the 

 migr;iti(Mis, pi)- .u\ of existing species and subspecies. 



In SMiiK- of these sciences, as in the case of ptilosis and pter)'- 

 lography tor example, not only must both sexes be studied and 

 compared, hut the young at all stages of their existence. Both 

 ptilosis and pterylography furnish very useful data to assist us 

 in the elassitication of Aves, and as exponents of a bird's topo- 

 graphical anatomy, for this purpose they are quite co-equal in the 

 mattei ot importance with the osteological system of th^e internal 



