394 SCIENCE PROGRESS. 



seen that the members of the lowest order, viz., the ostrich 

 and emu, are also the largest individuals upon which ob- 

 servations were made, whilst those of the highest order, 

 the Passeres, are the smallest. This apparent interdepend- 

 ence of size and body temperature may be a coincidence, but 

 it is nevertheless sufficiently striking to be worthy of mention. 

 Just as in morphology we know that, according to the law 

 of von Baer, the developmental history of the individual 

 to a certain extent recapitulates the developmental history 

 of the race, so also does a somewhat similar condition of 

 affairs exhibit itself in regard to the physiological question 

 under discussion. The embryonic warm-blooded animal, 

 which has descended from a cold-blooded ancestor, is itself 

 cold-blooded. Thus the physiological evolution of the 

 chick, in relation to its reaction to external temperature, 

 has been recently worked out very carefully by Pembrey, 

 Gordon and Warren. 1 On determining the respiratory 

 activity of eggs at various stages of incubation, these ob- 

 servers found that the developing chick, during the greater 

 part of its period of incubation, responded to changes of 

 external temperature like a cold-blooded animal. About 

 the twentieth to twenty-first day, however, an intermediate 

 stage was noticed, in which there was no marked response 

 in either direction. When the chick was hatched, this 

 neutral stage was succeeded by a stage in which it reacted 

 as a warm-blooded animal. On lowering the external tem- 

 perature, it now showed more active muscular movements, 

 and gave out an increased amount of carbonic acid. Guinea- 

 pigs, shortly after birth, were found by Pembrey ' 2 to react 

 in a similar way, but mice, rats and pigeons, which are born 

 blind, naked and helpless, react in the same way as cold- 

 blooded animals. On exposure to cold their carbonic acid 

 output is considerably decreased, and their body tempera- 

 ture also falls. The power of heat regulation was found, 

 however, to be well developed in mice by the tenth day 

 after birth, and in pigeons by the fifteenth to sixteenth day. 

 These animals had then attained their full and true status 



as warm-blooded animals. 



H. M. Vernon. 



l J. Physiol., xvii., p. 331. -Ibid., xviii., p. 363. 



