IQO Cleland and Johnston, Red Blood Cells. [isf"jan 



gave rise to the Batrachians and reptiles, and these latter to the 

 Aves ; the other with cells of decreasing size, giving rise to the 

 Teleostean fishes. In both cases we see that, with higher special- 

 ization, the red cells decrease in size. The interesting question 

 arises — Is this decrease in size merely a coincidence attendant on 

 favourable variation, or was it a necessity for such evolution ? 

 Did the ancient vertebrates of enormous size and reptilian 

 character possess extremely large red cells ? Did the extinction 

 of these forms in part depend on their inability to form smaller 

 red cells which could, with greater ease, supply oxygen uniformly 

 to all the tissues ? And why, in the oldest forms of vertebrates 

 that we have examined, do we find such large cells ? It would 

 be of great interest, in this connection, to examine the lampreys 

 as examples of another old group, and see whether there is 

 evidence that the original red cells were much smaller. Again, 

 what are the mechanical and physiological advantages or dis- 

 advantages of increase of size in the red cells ^ Large cells require 

 large capillaries, and these would, we presume, be fewer in number, 

 and hence oxygenation in distant cells would be less complete 

 than in those nearer the capillaries. Would increased efficiency 

 follow, therefore, decrease in size ? 



Batrachians. 

 The red cells of Batrachians vary a good deal amongst them- 

 selves, the average size being about i8 to 20, the extremes we have 

 met with being 14 and 23.5. No generic significance seems 

 attachable to the sizes. ^^ ^„^^ 



KEPTILES. 



Amongst the reptiles we again find considerable variation. 

 Snakes usually average about 17 to 20, with extremes of 15 and 

 21.5 ; lizards average apparently a little lower, from about 16 to 

 18, with extremes of 11. 5 and 20.5 ; whilst Chelonians average 

 about 18 to 20. The figures are very variable, but perhaps the 

 red cells of snakes and Chelonians are a little larger than those 



o^ ^'^^'^^- Birds. 



Most of our bird slides have naturally been made from Passerines. 

 In many of the other orders the number of specimens examined 

 is few, and this fact 'must be borne in mind when weighing the 

 conclusions we form. Amongst the largest cells we have met 

 with have been those of three members of the Ardeiformes (in- 

 cluded in the Ciconiijormes by Evans.)* These cells varied from 

 13 to 16 X 8 to 9. A single specimen of Sphenisciformes gave 

 14.5 X 9 to 10 ; one of the Podicipediformes, 13.5 to 14.5 x 7 to 9 ; 

 one Pelicaniformes, 14 x 7 to 8 ; five Charadriiformes varied from 

 II. 5 to 15, being usually 13 to 14.5 ; one Lariformes gave 12.5, 

 probably a low figure. A Megapode, belonging to the Galli- 

 formes, ran from 11 to 14, averaging nearer the latter figure. 

 Amongst the Coraciiformes, Dacelo and Halcyon ranged from 12 

 * Evans, " Cambridge Natural History — Birds." 



