298 Experimental Zoology 



The Union of Parts of Different Species 



As Darwin long ago pointed out, the union of parts of different 

 species has many points in common with the fertilization of the 

 egg of one species with the sperm of the other. In both cases the 

 combination can sometimes be successfully made, while in others 

 it cannot. Furthermore, the general statement may be made 

 for both cases that closely related species combine more readily 

 than those far apart, i.e. the results are more successful for unions 

 between closely "related" forms than between distantly "re- 

 lated" forms. Certain exceptions exist, however, in both direc- 

 tions. 



The principal experiments with animals are those of Joest 

 with earthworms ; of Born with tadpoles ; and of Wetzel with 

 hydra. 



It has been found to be more difficult to unite pieces of differ, 

 ent species of earthworms than of the same species ; nevertheless 

 Joest has succeeded in making a number of combinations of 

 different species. The most successful union, i.e. the one that 

 was most easily made, is that between Lumbricus rubellus and 

 Allolobophora terrestris. A compound worm of this sort lived 

 for eight months. Neither part showed any material influence 

 of the other half, although the blood circulating through the 

 worm went from one to the other. It may appear, in this case, 

 that since both- parts were completely formed at the time of 

 union, there is given no chance for the influence of the parts on 

 each other to manifest itself. If, however, a part of one of the 

 components is removed and a new part regenerates, the new 

 part must derive its nourishment from the materials in the blood 

 that come from both components. The possibility of such an 

 influence was tested in a compound made up of Lumbricus ru- 

 bellus and Allolobophora fcetida. The new head regenerated 

 from Lumbricus rubellus, that is the lighter in color. It was of 

 the same color as L. rubellus, and showed no influence of the 

 darker color of the other component, A. terrestris. 



Born has also found that it is more difficult to get pieces of 



