58 SCIENCE PROGRESS. 



two forms are clearly one species. Specimens have also 

 been seen with three toes on one foot and four on the 

 other. * These animals are so common that a statistical 

 study of their variation in nature should not be very 

 difficult to make. 



Among birds some of the most striking examples are 

 known, and have been the subject of a good deal of discus- 

 sion which, for the most part, has followed exclusively the 

 lines indicated above. One of the best known is that of the 

 species of Colaptes (a Woodpecker) inhabiting the United 

 States and Mexico. In this instance the facts have been 

 examined in considerable detail, especially by J. A. Allen, 

 (i) who has given a careful summary of the evidence de- 

 rived from the study of a large mass of material. As ap- 

 pears from his observations the case is briefly this. Omitting 

 forms with more restricted range, two species of Colaptes, 

 C. auratus and C. cafer, occupy nearly the whole of N. 

 America. A band of country 300 to 400 miles wide ex- 

 tending from British Columbia to Texas is common to the 

 two forms. North and East of this belt C. auratus is found 

 unmixed, while C. cafer in the pure form occupies most of 

 the country south and west of the common area. The dis- 

 tinctive characters are very striking, the most noticeable 

 being the following : — 



Besides these there are several less conspicuous differences, 

 one of the most singular being due to the fact that the 

 colours of the crown and throat are transposed in the two 

 forms, C. aurahis having a grey crown and a brown throat, 

 while C. cafer has a brown crown and a grey throat. 



In describing the " ever-varying combinations" of these 

 characters found in the area where the races overlap Allen 

 states that the specimens range from individuals which show 



1 1 have referred to the evidence in detail, Materials, No. 609. 



