500 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



of mice. By immunizing rabbits, they believe that they succeeded in differen- 

 tiating between Mus musculus and Mus bactrianus ; however, Mus musculus 

 could not be differentiated from Mus farvensis and Mus bactrianus behaved 

 like the Japanese waltzing mouse. These investigators concluded, therefore, 

 that Mus musculus is closely related to Mus farvensis, and Mus bactrianus to 

 the Japanese waltzing mouse. But, as Boyden remarks, this serological differ- 

 entiation of two so nearly related species is unusual and needs confirmation. 

 By reducing very much the quantities of antigen used for immunization Wolfe 

 was able to distinguish, by means of the precipitin reaction, between gray 

 squirrel and red squirrel, and also between ox and sheep, but not between 

 goat and sheep. 



By means of reciprocal immunization Boyden found the dog to be more 

 closely related to the pig than to the horse; still further distant from dog 

 were beef and sheep ; beef was nearest related to sheep, then followed horse 

 and pig and dog. However, Boyden is careful to state that these tests merely 

 indicate the relationship of these animals as they are constituted at present, and 

 that they do not necessarily correspond exactly to the phylogenetic evolution 

 of these species. Greater are the difficulties of distinguishing, by means of 

 the precipitin reactions, species among reptiles and amphibia, if rabbits are 

 used for immunization, although to some extent this, too, can be accomplished. 

 This method, supplemented by absorption tests, made it also possible to dif- 

 ferentiate between the hemocyanins present in the blood of various arthropods. 

 Within a certain range there existed a relation between the strength of 

 the precipitin reaction and the phylogenetic nearness or distance of the species 

 used. 



Different species of birds could be especially well differentiated serologically 

 (Defalco), more readily than mammals, amphibians or fishes; it is therefore 

 believed that birds represent a very homogeneous group. Blood serum, crystal- 

 lized egg albumin and lens of the eye, or hemoglobin, when used as antigens, 

 gave essentially similar results, except that in serum several proteins were 

 present, which introduced certain complications, while egg albumin and 

 hemoglobin represented essentially single proteins. Among invertebrates sharp 

 differentiations could be made between species belonging to different genera 

 as well as between others which belonged to the same genus. On the strength 

 of his tests of helminth species, Wilhelmi believed that it was possible to 

 define quantitatively the dilution of the antigen in the precipitin reaction 

 which was characteristic of species differences. The results obtained with the 

 precipitin reaction were also in conformity with the conclusion that echino- 

 derms, and especially holothurians, were more closely related to prochordates 

 than were annelids. In general, then, serological tests confirmed and made 

 more secure earlier conclusions based mainly on structural studies. 



The relationships of plants have been studied very extensively, by Mez 

 and others, by means of precipitin tests, and they have thus traced the 

 phylogenetic evolution of the vegetable kingdom. In contrast to the experi- 

 ments in animals, where usually body fluids or some of their constituents were 

 used as antigens, Mez employed for his determination, extracts of young, 



