THE PULSE-RATE IN VERTEBRATE ANIMALS jy 



artificial fattening up of these animals, thus increasing their 

 body-weight while their hearts, having little to do, do not keep 

 pace, finding it possible to supply the oxygen demanded 

 by increasing the frequency of beat. The animals with the 

 smallest hearts would be selected for such purpose by man just 

 because of their being the least active. By similar artificial 

 (though also unconscious) selection in the other direction, i.e. 

 of large hearts, that of the race-horse would be accounted for, 

 whilst 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 and 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, whilst 

 a veterinary surgeon informs me that in the ordinary horse its 

 range of variation is between thirty-four and forty only in health. 

 Hering's observations on the pulse-rates of forty-three rabbits 

 show that the modal resting frequency is lower than the average 

 frequency, thus suggesting in the case of the rabbit what 

 Muller'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 circumstances does not feel cold, if we may 

 judge from its behaviour. 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 



