lengths becomes still more striking in its trend upward as 

 the species also trend upward. 



Despite the difficulties inherent in a linear classification, 

 and the errors and conflicts in the incubation records, these 

 records show clearly that the incubation period becomes 

 shorter as the species mount the avian tree ; it is possible that 

 a rearrangement of the species according to some other clas- 

 sification might alter the curves radically and make other 

 interpretations necessary; it is equally possible that a re- 

 arrangement might strongly fortify the present conclusions. 

 Time and space forbid trying other classifications or com- 

 binations. It is obvious that the list of incubation periods 

 shows also that birds, in general, grow smaller as one follows 

 their life scale upwards, a fact which accords with the sug- 

 gestions as to "swiftness of life," size, and temperature. 

 One must not overlook the possibility that the character of 

 "swiftness of life" embraces the embryonic period (incuba- 

 tion) of life with birds, as it seems to do with the embryonic 

 period (gestation) of mammals. 



There is much to sustain the belief that the intensity of 

 metabolism and the speed of embryonic development are 

 correlated (155), but the details of this evidence cannot, 

 however, be given here. That size and temperature are more 

 or less correlated both with mammals and birds is unmis- 

 takable ; that size, temperature, and taxonomic standing are 

 all parallel is not so evident, yet it looks as though the 

 lower birds are the larger ones, and that the tendency to 

 become smaller and to have shorter incubation periods, as the 

 avian tree expands upward, is fairly obvious. There is some 

 geologic evidence pointing to the probability that ancient 

 birds were generally larger, a fact well brought out by the 

 large size of birds recently found in the California asphalt 

 deposits ; however, some living birds are small yet "low" in 

 degree, i. e., the kiwi, and some ancient birds were small yet 

 close to the reptile in some ways, the ichthyonis, for example. 

 This question of "lowness" or "highness" in birds, in the 

 present discussion, is a question of how far has a given 

 species journeyed away from its proto-avian stem, since it 

 seems probable that the farther a bird is from its primitive 

 ancestry, provided it does not later degenerate, the higher 

 will be its temperature. I doubt very much that the present 

 mainstays of taxonomy can alone measure this space between 

 pro-bird and super-bird. I believe that future students of 

 avian taxonomy wiU have to give more consideration, not^ 

 only to embryology, but also to bird physiology, in order to 

 correctly locate and plot the mile posts in a bird's taxonomic 

 journey. 



On the other hand, experimental evidence demonstrates 

 that the usual length of incubation may be elongated by dif- 



64 



