100 Lfypothesis of Structure of Germ Substance 
and so to secure the energy necessary to their activation, 
and thus to bring out in the cross certain characters of 
the ancestor which otherwise would not be found in the 
existing species in any of its ontogenetic stages. 
In this way then, by the arrest of development at the 
ontogenetic stage at which the respective germinal ele- 
ments of the two species begin to diverge from one an- 
other, can be explained in the most direct possible manner 
the above mentioned phenomena of atavistic reversion 
which all hybrids present. 
“The offspring of a cross of two such species,” writes 
Orr, “might therefore continue its development so long 
as the two inherited impulses were alike, but when the im- 
pulses begin to impel growth in opposite ‘directions, de- 
velopment must cease. This explains why the imper- 
fectly developed offspring of a crossed species resembles 
an ancestral form.” ° 
For example the distinct, colored, transverse stripes 
on the foreleg and shoulder of the mule, which in the 
horse and the ass are quite rare and usually very faint, 
arise in this way and must be referred to the common an- 
cestor of both species. From the crossing of certain races 
of pigeons arise birds which have the slate colored plu- 
mage of the wild dove, even though the races concerned 
in the crossing possess quite a different color. But it has 
been proven that these races branched off directly from 
the wild races. In the same way the mixed breeds of do- 
mestic ducks recall the wild ducks. And the hybrid of a 
German and Japanese pig is quite similar to a wild boar. 
The hybrids of Datura ferox and Datura laevis regularly 
have blue flowers instead of the white of their parents; 
Orr: A Theory of Development and Heredity. P. 230—231. 
