46 Chemical Basis of Genus and Species 



was due to the fact that this previous tissue transplan- 

 tation led to an increase in the number of lymphocytes 

 in the animal. The medical side, however, lies outside 

 of our discussion, and we must satisfy ourselves with 

 only a passing notice. The facts show that each warm- 

 blooded animal seems to possess a specificity whereby 

 its lymphocytes destroy transplanted tissue taken 

 from a foreign species. 



A lesser though still marked degree of incompatibility 

 exists also in lower animals for grafts from a different 

 species. 1 The graft may apparently take hold, but 

 only for a few days, if the species are not closely 

 related. Joest apparently succeeded in making a per- 

 manent union between the anterior and posterior ends 

 of two species of earthworms, Lumbricus rubellus and 

 Allolobophora terrestris. Born and later Harrison 

 healed pieces of tadpoles of different species together. 

 An individual made up of two species Rana virescens 

 and Rana palustris lived a considerable time and went 

 through metamorphosis. Each half regained the char- 

 acteristic features of the species to which it belonged. 

 It seems, however, that if species of tadpoles of two 

 more distant species are grafted upon each other no 

 lasting graft can be obtained, e. g., Rana esculenta and 

 Bombinator igneus. These experiments were made at 

 a time when the nature and bearing of the problem of 



1 The reader is referred to Morgan 's book on Regeneration (New 

 York, 1901), for the literature on this subject. 



