VOL. III. ] Recent Literature. 65 
theory to fact, but then a good and useful theory should not be too 
lightly discarded. CAs 
The North American Species of the Genus Colaptes, considered 
with Special Reference. to the Relationships of C. auratus and 
C. cafer. By J. A. ALLEN. Bull. Am. Mus. Nat. Hist., iv, 21-44. 
In the present paper Mr. Allen has undertaken a most careful and 
thorough investigation of the remarkable intergradation existing be- 
tween Colaptes auratus and C. cafer. His report is based upon 
tle examination of 785 specimens of the genus from North Amer- 
ica and the West Indies, representing all the known species and 
varieties inhabiting this region. The relationship of the two 
species under consideration is first discussed, and the characteristics 
and distribution of the various races given. More detailed atten- 
tion is then devoted to the intermediate birds, the conclusions ar- 
rived at with regard to them being stated as follows: ‘“‘ The facts 
elicited in the present investigation tend strongly to confirm Baird’s 
startling hypothesis of hybridization on a grand scale between 
Colaptes auratus and C. cafer, to account for the occurrence of birds 
presenting ever-varying combinations of the characters of the two 
species over the Plateau and Great Basin regions of the continent. 
None of the other hypotheses thus far advanced so fully, or, in fact, 
to any great extent, meet with the requirements of the case. In 
no instance do we meet with stages or methods of geographical 
variation at all comparable with what is seen in the case of C. aura- 
fus and C. cafer. The transition between geographic forms, how- 
ever diverse, is gradual and symmetrical, affecting all parts of the 
plumage equally and simultaneously, and is obviously correlated 
with changes in the physical surroundings; also, the differences 
between the most extreme forms are merely differences of degree. 
In the case of Colaptes, the essential differences between auratus 
and cafer are radical; they are, in fact, contrasting characters, and 
the intergradation is irregular, with all sorts of a symmetrical combi- 
nations of the characters of the two forms, and no correlation be- 
: $4 . ” 
tween their intergradation and the conditions of environment. 
Mr. Allen has, in fact, practically demonstrated the habitual hy- 
‘bridization of these two species, as Mr. Chapman has just succeeded 
in doing for the grackles. The bearing of this demonstration upon 
the infertility of crosses and the relation of color to sterility, as dis- 
cussed by Wallace in “ Darwinism,” is very important, placing the 
