310 GENETICS 



parental reproduction, in which new combinations of genes 

 are produced, then a period of multiplication by partheno- 

 genesis, In which the different combinations are multiplied 

 and subjected to the test of the conditions under which they 

 must exist. 



In all these methods of producing stocks of diverse ge- 

 netic constitution, later multiplying them by vegetative re- 

 production, it turns out of course that the different stocks 

 show diverse characteristics. Some are vigorous, some are 

 weak. Some are fitted out with full sets of efficient organs, 

 others are defective in certain ways. Some show high re- 

 sistance to unfavorable conditions, others lack resistance. 

 Some react In an effective manner to the conditions that 

 they meet, others do not. Some multiply vigorously and 

 rapidly, others weakly and slowly. All these characteristics 

 depend on the gene combinations that are present, and vary 

 as the combinations vary. In general terms, some of the 

 gene combinations are better fitted to existence under the 

 conditions than are others. 



The definitely weak and defective combinations quickly 

 disappear, while those that are vigorous and fitted to the 

 conditions survive and multiply. In time the population 

 consists only of the stocks with superior combinations. At 

 some later time, or under certain conditions, a new period 

 of biparental reproduction occurs, the new combinations 

 being made exclusively from the superior stocks that have 

 survived through the period of vegetative reproduction. 

 This process is continued for cycle after cycle. It seemingly 

 must have a tendency to gradually Improve the constitution 

 of the existing stocks. 



Relation to domestication. — Under natural conditions, 

 which are severe and not tempered to the weak combina- 

 tions, most of the new combinations produced die out, par- 

 ticularly the extreme and aberrant types, which are com- 

 monly not efficient. Only the best types continue to exist. 



