202 INBKEEDING AND OUTBREEDING 



qualities unquestionably having survival value in the 

 great majority of oases. It is a phenomenon so universal, 

 so uniform in its effect, it must have played an important 

 role during the course of evolution. Heterosis increasing 

 growth and fertility immediately, segregation favoring 

 adaptibility in the next generation, is a partnership of 

 some strength. An income for life and a trust fund ma- 

 turing for benefit of the children, what more could 

 one ask? 



Heterosis may even be pictured as the efficient cause of 

 sex survival. Some means of favoring union of dissimilar 

 spores occurred as a chance variation. Through the com- 

 bination of somewhat different qualities this new dual 

 product, the zygote, was better enabled to develop and to 

 reproduce. Its survival coefficient was high. The ten- 

 dency for union of spores persisted and became char- 

 acteristic of the species. Sex was established. 



This is a pleasing theoretical picture, and we do not 

 believe it is overdrawn, but it must be admitted that the 

 concrete evidence of a sexual union being immediately 

 beneficial in the lower organisms is not what might be 

 desired. Jennings ^^^ finds a marked slowing down of 

 the reproductive rate in the generations immediately fol- 

 lowing conjugation in Paramecium, with no beneficial 

 effect resembling heterosis, although he suggests that re- 

 combination may account for the best cultures. In fact, 

 nothing similar to heterosis has been found in unicellular 

 organisms. The lowest tjipe where distinct evidence of 

 the phenomenon has been discovered is in Trochelminthes, 

 cross-fertilization increasing size, vigor, viability and re- 

 productive rate in rotifers. But it would be strange 

 indeed if no such effect did occur in low forms when it is 



