have the shortest incubation periods ; so far as I am able to 

 grasp Gadow's words, he correlated this shortness of incuba- 

 tion with no definite physiologic process, and spoke of it 

 only incidentally in a discussion which related to the ques- 

 tion of a bird's being precocious or altricial. Arrigoni (12) 

 comes nearer to the final conclusion enunciated in this dis- 

 cussion, when he speaks of the length of the period of incu- 

 bation as being related to the vitality (sic) of the bird. I am 

 sure many others have .grasped this idea, but I have been 

 wholly unable to locate any written statements to such effect. 

 These, then, are all the hints, direct or indirect, which sug- 

 gest a possible relation of a bird's taxonomic standing to the 

 length of its incubation period, that I have been able to dis- 

 cover in the ornithological literature at my command. 



The evidence and data which I have been able to collect, 

 point to the idea that the true length of incubation yields to 

 change exceedingly slowly and with difficulty, much more so 

 than characters of more recent acquirement, as color, size, 

 etc. It is almost self-evident that the incubation periods of 

 birds have been gradually shortened and that they are still 

 slowly yielding, as in the past, to the influences outlined in 

 the foregoing, and will continue to do so until this process 

 of abbreviation becomes detrimental to the species. I am 

 firmly convinced that, once a given period becomes short- 

 ened, it remains so, becoming longer again only apparently, 

 and through such influences only, as slow or temporarily 

 suspend the embryonic development, and returns, either 

 in the next set of eggs, or with the first set of the next 

 generation, to the previous normal, after the warping influ- 

 ence ceases. Once a specie rises in "taxonomy,"' and has its 

 temperature coincidentally elevated, which shortens its in- 

 cubation period, this latter will remain constant under op- 

 timum conditions, even though the species later retrogrades 

 morphologically. I believe that in this case, the bird re- 

 mains physiologically stable, at the previous level, though 

 its co-existing morphologic level will have been lowered, and 

 that the bird will then have an incubation length which, 

 under the present theory, Avould appear shorter than its 

 morphology would predicate. On the contrary, a species 

 may "specialize" morphologically, appear as "high," yet re- 

 main at the pristine lower physiologic level, and have, as a 

 result, an incubation period longer than one would pre- 

 suppose for it, judging from its morphology. 



I am inclined to believe, as a result of this investiga- 

 tion alone, that there are physiologic as well as morphologic 

 levels to be considered in taxonomy, and that these levels 

 may differ in the same individual. I am convinced that there 

 is a large and profitable field to be explored, in this question 

 of the possible differences in physiologic and morphologic 



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