18 Zoologia: N. Y. Zoological Society. [Vol. 1 



An Australian trapper of large experience in taking these 

 birds says he has never noticed any variation from the normal 

 plumage in the respective habitats of the two forms, so this latter 

 conclusion would seem at least open to reasonable doubt. How- 

 ever, I have elsewhere in this paper discussed in detail the tax- 

 onomic side of a condition such as this. By the courtesy of Mr. 

 Seth-Smith I am able to present drawings of the intergradation 

 of the two birds. 



Part IV. — Experimental. 



The experiments detailed in the course of the present paper 

 are far from complete. They relate only to three species of birds, 

 but the similarity of result in each case and the possible signifi- 

 cance, in relation to certain factors of environment, in the evolu- 

 tion of birds as a whole, would seem to justify publication at this 

 early stage. This part of the contribution may thus be consid- 

 ered as only preliminary to future, more comprehensive re- 

 searches. The experiments were all carried on at the New York 

 Zoological Park with birds living in the collection of the Society, 

 and for the opportunity of making these studies I am greatly 

 indebted to my chief. Dr. William T. Hornaday. 



The early stages of the experiments were made under con- 

 ditions which rendered it impossible to make other than tempera- 

 ture records, but later, hourly readings with a hygrodeik showed 

 that, on the whole, the daily humidity was considerably greater 

 than that of New York City. The mean annual humidity of the 

 city is 73 per cent., while the average humidity to which the birds 

 under experimental observation were subjected was 84 per cent. 

 During the warmer months, from April to September inclusive, 

 the temperature averaged that of the city, 68 degrees, while from 

 October to March inclusive, it varied from 60 to 72 degrees. 



Hylocichla mustelina (Gmel.), Wood Thrush. — In the spring 

 of 1902 three young wood thrushes, nine days old and well feath- 

 ered, were taken from a nest and reared by hand. Both parents 

 were seen and were in every respect normally colored. Soon 

 after the young birds were taken, the parents built a second nest 

 and successfully reared another brood. The moult into the first 

 winter plumage was completed by the tenth of September, and a 

 little before this time, about the first of the month, two of the 

 birds were placed in a very humid atmosphere. One of them 

 died shortly afterward from some unknown cause, no lesions 

 being discernible, but the second bird lived two years, when it 

 met its fate at the beak of a pugnacious robin. In the early fall 



