520 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



to Landsteiner, donkey serum does agglutinate mule corpuscles, although not 

 to the same degree as horse corpuscles. Agglutination tests indicate, therefore, 

 that mule blood behaves essentially like horse blood, but in some respects 

 it shows the influence of the donkey. It seems, then, that in this species hybrid 

 the characteristics of the female parent predominate, suggesting the possi- 

 bility that we may have to deal at least with a partial cytoplasmic inheritance. 



Landsteiner wished to determine whether the blood groups which are pres- 

 ent in the horse, and lacking in the donkey, are transmitted to the mule. Von 

 Dungern and Hirschfeld had previously observed that the isoagglutinins of 

 the horse are transmitted to the mule. In the donkey, isoagglutinins are 

 not demonstrable. By means of immune sera and absorption tests Land- 

 steiner and Van der Scheer found in the erythrocytes of the horse more than 

 three kinds of isoagglutinable substances. The mule inherits from the horse 

 isoagglutinable groups, but not all the blood groups are equally transmitted 

 from horse to mule. The blood of the majority of mules belongs to the class in 

 which the serum contains agglutinins, and the corpuscles are not, or only 

 slightly, agglutinable. Not as many mules contain the isoagglutinable sub- 

 stances as do horses; this agrees with the fact that donkey blood contains no 

 blood groups with isoagglutinable erythrocytes. There is therefore, again, 

 an influence of the genetic constitution of the donkey noticeable in the mule 

 so far as the inheritance of the blood groups is concerned. But horse ag- 

 glutinogens are transmitted to mule corpuscles, which lead to heteroagglutina- 

 tion of mule corpuscles by donkey serum. Donkey serum behaves therefore in 

 the same way to a certain group of horse and mule corpuscles ; however, it 

 does not cause the agglutination of mule corpuscles to the same degree as it 

 does that of horse corpuscles, and there are more individual differences in 

 the erythrocytes of the mule than in those of the horse. Likewise, there are 

 differences between different donkey sera, but a given serum behaves in a 

 similar manner to horse and mule corpuscles. 



We find thus, that also in this instance mule blood resembles, on the whole, 

 horse blood ; but to a certain degree an influence of the donkey is noticeable 

 and it modifies the inheritance in the mule. We may furthermore conclude 

 that there are differences in individual mules. Of two mules, the one inherits a 

 characteristic from the horse, while another inherits the corresponding char- 

 acteristic from the donkey ; yet both are equally mules. The hybrid character 

 "mule" is therefore distinct from and independent of those individual char- 

 acteristics which differ in different mules, some of which resemble, as far as 

 this particular characteristic is concerned, more the horse, while others re- 

 semble more the donkey. The species differentials that distinguish horse, 

 donkey and mule are definite, although some of the mosaic characteristics 

 of individual horses and donkeys may vary, and the composite of these 

 characteristics also differs in individual mules. 



As to the analysis of mule blood by means of immune sera, Landsteiner 

 and Van der Scheer found that while it might be possible to use the precipitin 

 and complement fixation tests for the differentiation of the serum proteins 

 of these three types of animals, the differences established by such methods 



