DESCENT WITH CHANGE 367 



It will be recalled that chemical control by hormones plays an 

 important part in the coordination of the multicellular animal 

 into a working unit, especially in the case of such functions as 

 digestion, growth, and reproduction. Now it is significant that 

 the hormones seem to be largely if not completely interchangeable 

 from one species of Vertebrate to another. Thus a deficiency in 

 the insulin hormone in Man may be supplied from the pancreas 

 of a Fish or a Sheep. Obviously this strongly suggests that at least 

 certain of the chemical regulators have been common factors 

 from a remote ancestral period. 



But chemical differences as significant as anatomical ones have 

 been revealed by the type of crystals formed by the hemoglobin 

 of the blood. When orders, families, genera, or species are clearly 

 separated by anatomical criteria, the crystals are correspondingly 

 markedly differentiated. Thus from crystal form it is evident 

 that the common White Rat is the albino of the Norway Rat 

 (Mus norvegicus) and not of the Black Rat (Mus rattus) ; and that 

 Bears are more nearly related to the Seals than they are to Dogs. 



Again, there are other important chemical differences, not de- 

 terminable by ordinary chemical analysis, between the blood even 

 of closely related species, long known by the fact that the trans- 

 fusion of the blood of one species into another is usually attended 

 by physiological disturbances and often by death. It has been 

 found by innumerable transfusions and also by so-called precipitin 

 tests of the blood in vitro, that is outside the body, that the degree 

 of the reaction is in many cases proportional to the degree of re- 

 lationship of the species involved, as indicated by their classifica- 

 tion on the basis of anatomical structure. 



Thus, as one would expect, human blood shows by the precipitin 

 test closer chemical relationships with the blood of the highest 

 Apes than it does with that of the Old World Monkeys; closer re- 

 lationships with the blood of the latter than it does with that of 

 the New World Monkeys ; and closer with the blood of these than 

 with that of the Lemurs; and so on. Or, descending to the Reptiles: 

 paleontology indicates that there is a close relationship between 

 Lizards and Snakes and also between Turtles and Crocodiles, while 

 the reptilian ancestor of the Birds was probably more closely allied 

 with the latter than the former groups. These same relationships 

 are indicated by blood tests. 



Thus aside from a few startling exceptions, which further study 



