298 Experimental Zoology 
The Union of Paris 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, 7.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 foetida. 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 
