160 MUTATION 



ses between Oenothera biennis and 0. muricata, between O. 

 Biennis and 0. Hookeri and several similar species, the results 

 being sufficently similar to admit of taking one cross as a t}^- 

 ical example for the purpose of this discussion. The peculiar 

 type of inheritance first discovered by de Vries in these l^y- 

 brids was confirmed by several authors, e.g. the cross Biennis 

 X muricara was repeated by Davis, with the same results as 

 obtained by de Vries. 



The hybrids which have Biennis as the mothei*, differ from 

 those which have it as the father. Cases of unlikeness of recip- 

 rocal hybrids are rare and they show, that the germ-cells 

 given off by a certain species as the father are not genot}^)- 

 ically identical with those produced by that same species as the 

 mother in the two crosses. A classical example of difference be- 

 tween reciprocal hybrids is that of the horse-donkey hybrids. 

 Here, the mule, the product of a male ass and mare, is sup- 

 posed to differ from the hinny, the reciprocal hybrid. As Gold- 

 schmidt pointed out however, the cross in both cases is not one 

 between the same two breeds. Mules are generally produced 

 from heavy draught-mares, Percheronnes, or Mulassi^res or 

 Belgian, and some very large donkey of the Poitou, Catalonian 

 or other Spanish breed. The reverse cross however, is only made 

 where donkeys are plentiful and cheap, and mostly very small, 

 such as in Mexico or Algeria, and the stallion used is very often 

 some small nondescript pony. If however, the same breeds are 

 used in cross-breeding, that produce the valuable mules, hy- 

 brids are produced which are practically identical with the re- 

 ciprocal hybrids. Hybrids from a heavy draught-horse stallion 

 and good, large Spanish Jennies are hard to distinguish from 

 mules bred in the ordinary way. 



In the instance of the Oenothera however, the parents used 

 for the reciprocal crpsses, are the identical plants, and the 

 hybrids from Biennis ovules and Muricata pollen are unmis- 

 takably different from those developed out of ovules of the 

 same Muricata plant with pollen of the Biennis plant used in 

 the reciprocal cross. 



