206 Mr. A. R. Wallace on the Natural Ananyement of Birds. 



It is intended that the distances between the several names 

 shdtild show to some extent the relative amount of affinity ex- 

 isting between them ; and the connecting lines show in what 

 direction the affinities are supposed to lie. Ey referring to the 

 diagram it will be seen that there are seven families placed 

 close together, forming a central mass. Ik-yond the Trogons at 

 some distance come the Goatsuckers and Swallows, while at the 

 greatest possible distance from each other are the Hornbills and 

 the Hummers, the former having a distant affinity to the King- 

 fishers, the latter to the Swallows. 



AVe may here mention that it is an article of our zoological 

 faith, that all gaps between species, genera, or larger groups are 

 the result of the extinction of species during foi-mer epochs of 

 the world^s history, and we believe this view will enable us more 

 justly to appreciate the correctness of our arrangement. For in- 

 stance, let us suppose that the gaps shown in this diagram have 

 been all filled up by genera and families forming a natural trans- 

 ition from one of our groups to the other, and we shall be able 

 to judge whether our arrangement will agree with such a sup- 

 position. Thus, if the space between the Kingfishers and Horn- 

 bills has been filled up by a natural succession of families, we 

 can see that the change must have been to heavier, larger, and 

 larger-billed birds, and we see such a change begun already from 

 the Jacamars to the Kingfishers. So from the Goatsuckers to 

 the Swallows the change is to smaller, stronger-winged, thicker- 

 skinned, and brighter-coloured birds, — exactly the kind of change 

 which continued on will lead us to the Hummers. On the same 

 principles we may conclude, that as the change from the Jacamars 

 to the Rollers is to stronger-legged birds which do not feed 

 solely on the wing but also on the ground, so the same change 

 continued on would lead us to true Passeres in which the pecu- 

 liar Fissirostral characters altogether disappear. The Coraciadce, 

 therefore, are either an extreme Passerine form of the tribe, or 

 else form a transition by direct affinity to the Passeres. 



The method of representing affinities here adopted we believe 

 to be of the highest value. It is founded on the method sug- 

 gested by the late Mr. Strickland, and which we believe Dr. 

 Lindley has been the first naturalist to adopt, namely that of 

 placing to the right and left of every family or other group the 

 names of those to which it is most nearly allied. But this alone 

 conveys no idea to the mind, especially in an extensive group, 

 till represented by a diagram, the most convenient way to con- 

 struct which we have found to be as follows. Suppose you have 

 a family of a dozen genera which you wish to arrange ; first write 

 down the names of the genera in any order, and right and left of 

 them those to which you believe them to be allied most closely. 



