ioo Hypothesis 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." 66 



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. 230231. 



