CAUSE OF HYBRID VIGOR 169 



ing and outbreeding with the phenomena of Mendelian 

 heredity was a great step forward. It went as far as it 

 was possible to go at the time it was devised, and it is 

 capable of interpreting all the facts to-day. But it held 

 some disadvantages. The assumption of a physiological 

 stimulation arising from the interaction of different 

 hereditary factors was not altogether satisfactory, for it 

 locked the door on any hope of originating pure strains 

 having as much vigor as first generation hybrids. For- 

 tunately, the development of Mendelian heredity has been 

 such that this part of the hypothesis can be superseded. 



The basis for this hypothetical stimulation was seen in 

 the fact that fertilization is usually necessary to start the 

 development of the egg. In most cases, without the union 

 with the sperm, the egg cannot divide and development 

 is prevented. The reaction of the different substances 

 brought together at fertilization stimulates cell division 

 and starts development. This made it reasonable to as- 

 sume that when the egg and sperm differed in hereditary 

 factors stimulus to development was increased and con- 

 tinued throughout the growth of the resulting organism. 

 According to the view of G. H. Shull and of East, it was 

 the interaction of different elements in the nuclei that 

 produced the stimulation. A. F. Shull, 198 on the other 

 hand, assumed the stimulation to result from the inter- 

 action of the new substances brought in by the sperm with 

 the maternal cytoplasm. In his opinion the stimulation 

 might persist for a time even after homozygosis was at- 

 tained, because foreign elments brought in by the original 

 cross would still remain to react with the cytoplasmic 

 matter. Moreover, it was further assumed that the stim- 

 ulation might decrease in long-continued asexual propa- 



