just enunciated, it becomes necessary to arrange the species 

 affected, according- to some reasonably acceptable classifica- 

 tion. It was Avith hesitation that I approached this question 

 of classification, and when the data finally had to be classi- 

 fied, I wrote to several of my professional ornithological 

 friends, requesting their opinion as to the best present avian 

 classification. All, as a unit, expressed themselves virtually 

 in the w^ords of one who said, ''There is no he-^f classifica- 

 tion," saying further that the arrangement of avian tax- 

 onomy as given by Gadow, and slightly modified by Knowl- 

 ton (10), was as good as any, which is the one followed in 

 listing the incubation periods, and in the secondary tables 

 and lists given in this study. 



In speaking of a species "lowness" or "highness,'' I do 

 not overlook the many difficulties attendant upon a deter- 

 mination of these levels. What I really would like to know, 

 viewing the question from the standpoint of this discussion, 

 is how far has a given species traveled from its proto-avian 

 ancestors, and not how much has it specialized. No linear 

 classification can show this, however perfect our knowledge 

 may be, nor can it exhibit the true positions of species in 

 one family in any given order as levelled with species in a 

 family in another order, nor yet can it give an adequate idea 

 of the relation of species of different families in the same 

 order. Many inconsistencies and contradictions appearing 

 under the present explanation of what controls the f?'ue 

 length of incubation are possibl^^ due, not only to lacunae in 

 our knowledge, and to errors in the records of incubation 

 lengths, but also to the shortcomings of a linear classifica- 

 tion. In other words, incorrect taxonomy, and the inability 

 to properly depict the relation of species in one order to 

 those in another, result in what appear to be severe disloca- 

 tions of incubation lengths from positions one would assign 

 to them under the present explanation. 



Within natural groups (or families), the incubation 

 lengths should be more or less characteristic because the 

 members of such groups have diverged but little, inte?' ,se, 

 and being less plastic than many other characters, the incu- 

 bation length, under these circumstances, would diverge 

 still less, resulting in a condition substantiating Evans' (2) 

 first conclusion, to-wit, that natural groups (or families) 

 have incubation periods more or less characteristic to such 

 groups, a conclusion borne out by the incubation length data 

 given in Table Xo. 1 ; the gradations in some families be- 

 tween the accepted taxonomic positions assigned to the spe- 

 cies correspond surprisingly closely with the variations of 

 incubation length of the same species, especially if in such 

 families the largest species are ranked as lowest. If all 

 families be so arranged, the undulating curve of incubation 



63 



