408 Mr. A. R. Wallace on the Arrangement of the 
found to accord with a striking anatomical feature, and to 
mark out an important natural group of families, it was not 
applied to the more extensive series of families which remained, 
and whose arrangement has continued to this day in a most 
unsettled state. Hardly any two ornithologists agree as to 
the order in which these families most naturally follow each 
other; and even in the most recent classifications the pecu¬ 
liarities of the first primary are deemed of so little importance 
that birds which markedly differ in this respect are sometimes 
placed in the same or in adjacent families or, even, genera. 
After repeated attempts, during many years, to group natu¬ 
rally the families of Passeres, I have recently come to the con¬ 
clusion that variations in the number and development of the 
primary quills indicate deep-seated affinities, and furnish the 
best, because the most simple and practically convenient, 
means for the further subdivision of this extensive Order. 
The fact that similar peculiarities of wing-structure run 
through whole series of families which are undoubtedly re¬ 
lated, is a clear indication of the importance of these cha¬ 
racters ; and we shall, I think, find that if we follow them 
out cautiously, and give due weight in doubtful cases to other 
proofs of affinity, we shall be led to a grouping of this vast 
and complex mass of birds which avoids many of the diffi¬ 
culties that have hitherto beset their classification, and ac¬ 
cords in a remarkable manner with the main features of their 
geographical distribution. 
Four types of wing are distinctly recognizable among the 
Passeres. First and most numerous are those with 10 pri¬ 
mary quills, the first of which is greatly reduced in size; then 
we have the American series, in which the first primary is well 
developed; and a small Old-World series, in which it is rudi¬ 
mentary ; and lastly a series in which the first primary is 
aborted, and which thus possesses only 9 primaries. These 
differences may be tabulated as follows; but it is found most 
convenient to arrange them in the order of the appended 
numerals, as we thus pass most easily from one series to 
the other, and that order best accords with existing arrange¬ 
ments :— 
