502 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1910. 



being the least active. By similar artificial (though also uncon- 

 scious) selection in the other direction, the relatively large heart of 

 the race horse would be accounted for, while in the case of the deer 

 and the bat, which are the only other mammals, of those in which 

 relative heart weight has already been determined, with so large a 

 heart as the race horse, the same end has been achieved by natural 

 selection. 



That frequency of beat in a resting condition, as well as relative 

 heart size, furnishes material (whether it is used or not) for natural 

 (or artificial) selection to work on is a fact of common experience 

 so far as man is concerned. I have found it to vary between 45 arid 

 90 per minute in quite healthy people. The extent of the range 

 seems to be very different in different species, thus in the mouse it 

 varies between 520 and 810 per minute, in the rabbit between 123 and 

 306 per minute, while a veterinary surgeon informs me that in the 

 ordinary horse its range of variation is between 34 and 40 only in 

 health. Hering's observations on the pulse rates of 43 rabbits show 

 that the modal resting frequency is lower than the average fre- 

 quency, thus suggesting in the case of the rabbit what Miiller's 

 observations did in the case of man, that it has come from a slower 

 pulsed and larger hearted race. 



Can we go further than showing that variations in frequency exist 

 to be selected from if need be, and indicate also the method by which 

 the heart in birds and mammals has succeeded in adapting itself to 

 the needs of the organism? We know that regulation of heat in 

 every individual warm-blooded animal is brought about by the 

 agency of the central nervous system. We know also that a warm- 

 blooded animal never is cold, although it feels cold when brought into 

 cold surroundings, while a so-called " cold-blooded " one which really 

 does become cold under similar cicumstances does not feel cold, if we 

 may judge from its behavior. We find that instead of making the 

 attempt to produce more heat to counterbalance the loss, by eating 

 or moving about, it refuses to do either of these things in the cold. 

 It will not even choose the warmest place and so prevent as much 

 loss of heat as possible. I kept a young crocodile for some months 

 in a long trough so arranged that one end but not the other might be 

 heated from outside. It was so heated every night when the weather 

 was cold, but the crocodile was found indifferently in any part of 

 the trough in the morning, until at last one night in a somewhat 

 longer spell of cold weather it died at the very farthest extremity 

 of the trough from the warmed part. It could have been in a sur- 

 rounding temperature of 8° C. had it liked; it chose one that was 

 hardly above freezing point and died there. A warm-blooded ani- 

 mal, feeling the cold, would have made every effort both to prevent 

 loss of heat and to produce more heat, and even without effort it 



