HYBRID VIGOR OR HETEROSIS 151 



a return to the condition of the original stock before in- 

 breeding was commenced. Some combinations regularly 

 give greater increases than others, but in every case such 

 differences are small as compared with those between the 

 crosses and the inbred parents. 



Although, in the main, reciprocal crosses give about 

 the same result, some variation in this respect is habitu- 

 ally shown. In general, there is a correlation between the 

 yield of the better parent strain and the yield of the cross. 

 The crosses in which strain No. 1-6 has been used as the 

 female parent have regularly given the highest yields, and 

 this strain is the most vigorous and productive of the 

 four inbred Learning strains used in our illustrations. 



In a comparison of crosses between inbred strains of 

 maize with ordinary outcrossed varieties the inbred hy- 

 brids are handicapped because they have to start from 

 small, poorly developed seeds. This handicap is brought 

 out clearly by a comparison of second generation plants 

 grown from self-fertilized seed produced on vigorous 

 hybrid plants, with hybrid plants grown from seed pro- 

 duced on inbred plants. The first generation starts off 

 poorly, as shown in the accompanying illustration (Fig. 

 33), but soon catches up and passes the second generation. 

 At maturity the second generation is shorter and less 

 productive, although it has a much greater variability. 

 The third generation from self ed plants of this particular 

 cross has been grown, and there is still further loss of the 

 stimulation which is at its maximum in the first genera- 

 tion. On continued inbreeding these families presumably 

 would exhibit a continuation of the same course of reduc- 

 tion in size, vigor and variability shown in the original 

 inbreeding experiment, until homozygosity was again 



