Chapter 4. 



The Organismal Differentials of Hybrids 

 Between Nearly Related Species 



In all conditions which we have studied so far, in which serological 

 methods have been used for the analysis of relationship, we were able 

 to compare the results with those obtained by means of transplantation. 

 There is, however, one type of relationship in which such a comparison 

 between these two methods of investigation is not feasible at present. The 

 mutual relations of hybrids between nearly related species, as well as 

 the relations between the hybrids themselves and their parents, have been 

 analyzed by serological methods, but only in a rudimentary manner by 

 transplantation methods; the number of experiments representing the latter, 

 made by Schultz, is very small. As Jacques Loeb has pointed out, a comparison 

 of the species characteristics of an F x hybrid with those of the father and 

 mother species should give an indication as to whether the rules of Men- 

 delian heredity apply to the transmission of species characters, or whether 

 hereditary transmission in this case takes place through the cytoplasm of the 

 egg. If it takes place through the cytoplasm, the hybrid should resemble the 

 mother species, but not the father species. Jacques Loeb suggested that it 

 might also be possible to determine this question by comparing the characteris- 

 tics of hemoglobin crystals of mule with those of horse and donkey. But the 

 measurements of Brown showed that the mule crystals were outside the 

 range of figures found for horse as well as for donkey, although they were 

 somewhat more nearly related to those of the donkey. Thus crystallography 

 did not help in solving this problem. 



I. In order to test the relationship between horse, donkey, and the hybrid 

 between these two species, the mule, use was made by Walsh of the presence 

 of preformed hemolysins and hemagglutinins in these three types of sera 

 acting on the various kinds of erythrocytes, while Landsteiner employed for 

 this purpose immune agglutinins. The findings of Walsh were as follows : 

 (a) Horse serum does not hemolyze horse, donkey or mule erythrocytes. Don- 

 key serum hemolyzes both horse and mule erythrocytes in a large percentage 

 of cases. Mule serum does not hemolyze horse or mule erythrocytes, (b) 

 Horse serum agglutinates neither horse nor mule erythrocytes. Donkey serum 

 agglutinates the erythrocytes of the horse in a high percentage of cases, but 

 does not agglutinate the erythrocytes of the mule. Mule serum does not ag- 

 glutinate horse or mule erythrocytes. From these observations it may be con- 

 cluded that mule serum behaves like horse serum, and mule corpuscles behave 

 like horse corpuscles rather than like donkey corpuscles, except as far as 

 the agglutinating action of donkey serum is concerned. However, according 



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