TABLE NO. 5 (78) 



If, now, one examined these various egg weights some 

 interesting conditions appear: First as to isolated cases of 

 comparisons, onei notices that the ostrich and kiwi have equal 

 lengths of incubation, yet the ostrich egg weighs three and 

 three-quarters pounds, while that of the kiwi is less than a 

 pound ; a pigeon's egg weighs five times more than an Eng- 

 lish sparrow's (in fact the pigeon's egg is heavier than the 

 sparrow itself) but there is only three or four days' differ- 

 ence in the incubation periods of these two species ; the eggs 

 of the yellow-billed tropic bird and those of the guinea hen 

 are almost identical in weight, nevertheless these two species 

 h!ave incubation periods differing in length at least four 

 days. If one make a curve of the incubation periods, ar- 

 ranging them according to the weights of the eggs, the 

 heaviest first, it can be noticed that again there is a general 

 tendency for the incubation length to shorten as the egg is 

 lighter, but there, are so many sharp deviations from the ex- 

 pected regularity that one suspects that there is another 

 factor, not influenced by the 'egg weight, which is at work 

 in fixing the length of incubation or at least such a second 

 factor is working in conjunction with the effect of egg . 

 weight. This possible second factor will be discussed in the 

 later portions of this study. Suffice it here to say, that, 

 while this list of egg weights is one altogether too short on 

 which to base a final judgment, which must be held for the 

 time in abeyance, these curves and such other considerations 

 as just outlined compel me to believe that egg weights are 



35 



