IN RELATION TO ZOOLOGICAL DISTINCTION. 35 



1886, L, 75; Nasse, Wagner's Handworterbuch, Das Blut, i, 134; Gscheidlen, 

 Physiologische Methodik, 1877, 328; Sherrington and Copeman, Journal of 

 Physiology, 1893, xiv, 52; Harris, Journal of Physiology, 1903, xxx, 319) 

 show in a general way at least that differences exist in the bloods of differ- 

 ent species which indicate zoological distinctions. Examining the records of 

 tables 9 and 10 it will be seen that specific gravity is higher in warm-blooded 

 than in cold-blooded animals; higher in birds than in mammals; higher 

 generally in herbivora than in man and monkey, rodents, and carnivora; 

 highest in birds, and probably lowest in carnivora. The high standard 

 in the frog and the snake are particularly noteworthy. Among herbivora, 

 the specific gravity of the ass and sheep are particularly low; that of the 

 cat is distinctly lower than that of the dog; that of the rabbit decidedly 

 lower than that of the mouse, rat, and guinea-pig. 



There is not in the differences of specific gravity the quantitative demar- 

 cation between warm-blooded and cold-blooded animals that was found to 

 exist in the proportions of blood to body-weight, nor does there appear 

 any approach to the class distinctions there noted. 



The cause or causes of the differences in the specific gravities of the 

 bloods of different animals are of course of much more importance than the 

 specific gravity per se, because, while we may in any two or more instances 

 find the same specific gravity in related or unrelated members of an order 

 or class, etc., for instance, the horse and pig, or the bullock and goat, or the 

 rabbit and cat, etc., thus expressing the same percentage of solids, gravity 

 gives no indication as to how those solids are constituted that is, as to 

 how they may vary in kind and proportions in the different bloods. Since 

 hemoglobin and other proteins represent nearly the whole of the solids, we 

 infer that differences in specific gravity express somewhat closely corre- 

 sponding differences in the percentage of one or the other, or both, of these 

 constituents. In fact, in human blood, under normal and certain abnormal 

 conditions, the relationship between specific gravity and the percentage 

 of hemoglobin is so constant within narrow limits that the clinician makes 

 use of specific gravity tables which indicate quite accurately the percentage 

 of hemoglobin present. Thus, a blood having a specific gravity of 1.059 

 (water taken as 1.000, as determined by the hydrometer) will be found to 

 contain about 14 per cent of hemoglobin; at 1.056, about 11.2 per cent; 

 at 1.052, 9.8 per cent, etc. 



While there is thus an unquestionable relationship between these 

 factors in human blood, it does not follow that if in different species we find 

 the same specific gravity there will be the same or even nearly the same 

 per cent of hemoglobin. Thus, while the specific gravities of the bloods of 

 the bullock and pig are the same, the hemoglobin percentages are 10 and 14 

 respectively; the specific gravities of the blood of the rabbit and cat are 

 about the same, but the percentages of hemoglobin are 12.3 and 14.3 respec- 

 tively. The blood of the dog has a higher specific gravity than that of the 

 cat, but a lower hemoglobin content; and birds have the highest specific 

 gravity of all animals examined, yet a relatively low proportion of hemo- 

 globin. In the bloods of different species the protein content of the plasma 



