(120) believes that the emu is a lower and a more primitive 

 bird than the ostrich, and his classification is supported by 

 the position taken in this paper, i. e., tliat the longer is the 

 incubation period, the lower is the bird. The secretary bird 

 is believed to be a primitive survival, not greatly changed, 

 from its pro-hawk-heron ancestor ; if this be correct, a long 

 incubation period would be predicted, viewed from the 

 "ascent theory," a period as long, or longer, than all of its 

 near relatives, and such is the case (barring one or two 

 questionable exceptions). These and similar questions must 

 be decided by experimental work ; they are but a few of the 

 many fascinating ones uncovered by a study of the length of 

 incubation. 



It is possible that many conflicts, and inconsistencies in 

 this new theory, will be cleared up when the present incuba- 

 tion records are checked and corrected by new observations, 

 undertaken to eliminate, or allow for, the conditions which 

 apparently increase or decrease the true length of incubation, 

 and when a method of recording avian classification will 

 have been devised which does not project the vision of the 

 mind's eye along a single line, and which will graphically 

 portray the relation of genera to genera, irrespective of fam- 

 ily, or order. Perhaps such a three- dimension demonstration 

 scheme is impossible of attainment. 



The chances of any given explanation being correct are 

 larger if the results it foreshadows can be shown to be 

 beneficial ; if the bird's taxonomic position, temperature, size, 

 and length of incubation be correlated, it is pertinent to ask, 

 has the combination been beneficial to the race? It would 

 seem so, judging from actual experience in nature, since so 

 large a number of existing species are of the highest levels, 

 have diminished in size, have acquired short incubation 

 periods, have developed high temperatures and have con- 

 comitantly flourished and multiplied. Do short incubation 

 periods prove helpful to the species ? Is there any advantage 

 to the robin that it gets its young out of the nest and able, 

 more or less, to shift for themselves in four weeks from the 

 laying of the eggs, while a goose, a turkey, or a red-tail 

 hawk, in the same time is able to complete only the em- 

 bryonic development of its young, and has still ahead of it, 

 the long post-embryonic period of development, and all 

 the care incidental to it, plus such post-nidicular time, during 

 which it must give more or less care to its young? Is there 

 any benefit to a species to be able to put two broods of young 

 into the field, while another species can, in the same time, put 

 but one? It seems to me that these questions can be an- 

 swered only in the affirmative. If it be true that the number 

 of eggs laid in a set is in direct ratio to the dangers en- 

 countered by the species (110) , it also would seem reasonable 



73 



