tion. It is stated by Chapman (65) that individuals of the 

 same species living in the Tropics lay fewer eggs than do 

 those in Northern regions, and it would be of value and help 

 if one knew if this difference in the number of eggs is ac- 

 companied by a change in the incubation length, all of which 

 still remains to be investigated. At the same time, it is nec- 

 essary to call attention to the fact that the snow bunting's 

 length of incubation is much out of relation to the periods 

 of other fringilline birds, a fact possibly due to zone, or to 

 errors of observation, and that Clark's crow has an incuba- 

 tion period which is rather long in comparison with others 

 of its family, or its taxonomic position. One of its con- 

 geners (the Canada jay) nests during the snows of winter, 

 and at high altitudes, too, without having an unusual length 

 of incubation. 



Some records of incubation lengths state that the eider 

 ducks (137) have a shorter period than does the domesti- 

 cated duck in warmer climate. Such statements remain to 

 be substantiated, but still add force to the belief that, in the 

 future, incubation lengths must be studied with an eye to 

 eliminating such influences as slow down and prolong em- 

 bryonic development, and thereby distort the true incubation 

 period. 



Heinroth (162) believes that the short periods of 

 Mereca penelope, Chen rossi and Dafila acuta (which he 

 gives as twenty- two to twenty-three days) is due to the short 

 northern summer; granting that these periods are correct, 

 why is the period of the teal in more southern latitudes no 

 longer ? Is the period of the Emperor penguin nearly fifty 

 days, because it occurs in the dark, winter months? I think 

 not. 



Climate 



Information on the possible effect of climate on the in- 

 cubation length is absolutely lacking, so far as the writer 

 has been able to determine. It is conceivable that a species 

 living in a moist climate might be unsuccessful in incuba- 

 tion, if suddenly made to live in an arid region (or vice 

 versa), because of its egg's structure having been adjusted 

 for generations to a given average humidity, which adjust- 

 ment fails in the new locality. While such a possibility is 

 extremely remote under natural conditions, a similar result 

 does occur with hens' eggs in artificial incubators in Colo- 

 rado (a semi-arid region), the writer having been given to 

 understand that failures to hatch in incubators, in Colorado, 

 are frequently due to egg dessication. It would shed some 

 light on this question if one knew if there were any differ- 

 ence in the lengths of incubation of white heron in India 

 and in Europe, because of the two-fold effect of differing 

 egg size, and climate. 



41 



