THE PULSE-RATE IN VERTEBRATE ANIMALS 75 



of the golden oriole it varied between r8 and 2"6 per cent., 

 whilst in seven of the curlew-sandpiper it varied only between 

 i'6 and 2"o per cent. In man, according to the determinations 

 made by Bergmann from thirty-six people in whom death was 

 accidental, the variation may be from o'43 to 075 per cent. 

 MiiUer (2a) dealing not with the weight of the whole heart but 

 only with that of the musculature, in percentage of body-weight, 

 in a large number of individuals who had died of different 

 diseases, shows by his tables that in about 800 people dying 

 between the ages of thirty and sixty, this varied from 0*26 

 to 0*89 per cent, and further that the percentage weights of this 

 musculature did not vary symmetrically about a mean, but 

 asymmetrically about a mode (i.e. the percentage weight of the 

 greatest number) and in such fashion that the mode (o'49 to o'5o 

 per cent.) was nearer to the relatively small hearts than to 

 the large ones, suggesting that the heart in man is becoming 

 relatively smaller. The suggestion that man's ancestors were 

 larger-hearted is perhaps supported by the fact that in infants 

 the modal ratio is about o'6 per cent, and even in children from 

 four weeks to three years of age it is further in the direction of 

 the large heart than in the adult, being about 0*53 per cent. 

 But we have to be careful in drawing such inferences from data 

 which cannot be determined in the living, since we do not know 

 in how far the heart-ratio affects the death-rate, a point which 

 Miiller, who interprets his tables in a way very different from 

 that which is here suggested, seems to neglect. However this 

 may be, we have ample evidence that in man, as in other 

 mammals, in birds, and so far as we know also in the lower 

 vertebrates, the material is there to be selected from should 

 it for any reason become advantageous for a species to alter its 

 heart-ratio in the future as it has probably done in the past. 

 With regard to the past it seems probable that such variations 

 were used as material for selection before they became correlated 

 with frequency of beat and that it was with the size of the heart 

 more or less already determined that this frequency, which is 

 also known to be variable in individuals, began to be used when 

 it began to be advantageous to be independent of external 

 temperature, owing perhaps to a change from an equable to 

 a variable climate. In the present state of our knowledge it is 

 difficult to point to any advantage which might accrue to any 

 species of poikilothermic vertebrate from having a particular 



