in hand. Another way to put this view of the swiftness of 

 life is that the rapidity of growth from birth to maturity 

 is proportionate to the metabolic intensity, which last is, as 

 said before, directly related to size ; the elephant takes years 

 to mature, the mouse can breed at the age of a few weeks. 

 This difference in the speed of life process in large and 

 smaller animals is not based on theory, but is demonstrable 

 by experimental methods (156), all of which show that the 

 metabolic intensity is proportionate to the animal surface 

 area, which is relatively larger in the smaller mammals 

 (148). Hand in hand wjth this skwness or swiftness of 

 metabolic processes are found corresponding differences in 

 the animal's physiology, particularly in the respiration, 

 heart rate, temperature, and possibly in the embryonic metab- 

 olism. There are some indications at hand suggesting that 

 this relation of swiftness of life and body size prevails in 

 the embryonic, as well as in the post-embryonic stages of 

 existence. There is suggestive evidence that the larger the 

 mammal the longer is its gestation period; with the mouse 

 it is three weeks and with the elephant at least eighteen 

 months, and animals intermediate in size show a fairly well- 

 defined intergrading in the gestation period length. Pem- 

 brey's remark that "the temperature of the smaller mam- 

 mals and birds is often higher than that of the biggest" 

 (156) gives a logical introduction to the question. Does this 

 relation of size and swiftness of life in mammals obtain also 

 with birds? Simpson (166) has definitely answered it in 

 the affirmative, so far as the hen is concerned, for he stated 

 that the large and most lethargic birds (i. e., of hens) had 

 a much lower temperature than the smaller and more active 

 ones. I am convinced that a thorough study of avian physi- 

 ology will show the same variation in the swiftness of life 

 in this class as is demonstrated in mammals. It seems quite 

 likely that increasingly intense metabolism and steady dimi- 

 nution in size of birds are related, and that the intense metab- 

 olism finds expression in many other ways in the function 

 of birds. For example, I have found a house wren's respira- 

 tion to be 160 per minute, and that of a house finch to be 100. 

 Rapid respiration and fast heart rate are always (in health) 

 co-existent; if one take the respiration: pulse rate index of 

 man, ?'. e., one to four, and apply it to these two species, it 

 would show the wren to have a pulse of 640, and the finch, 

 one of 400 beats per minute. Since there seems little known 

 about a bird's minute physiology, it might be hazardous to 

 assume that the human index can be correctly applied to 

 birds. However, there can be no question as to the amazing 

 rate of the heart beat in birds; if one hold. an English spar- 

 row or any small bird loosely but securely and gently in the 

 hand, and apply it closely to one's ear, the bird's heart, as it 



