334 
Journal of Agricultural Research voi. xxvn, No. 6 
had been selective survival of the heterozygous seeds or seedlings, the 
percentage of hybrids in the adult population would have been higher 
than the percentage of ovules fertilized by the unlike pollen. In that 
case the data in Table I would indicate less than the full measure of 
selective fertilization in favor of the like pollen. 
Four seeds were planted in each hill but in many of the hills only one 
or two plants survived. If there had been selective survival in favor 
of either the homozygotes or the heterozygotes, it seems probable that 
the percentage of hybrids obtained where conditions were relatively un¬ 
favorable (hills containing one or two plants) would differ from the per¬ 
centage obtained where conditions were more favorable (hills containing 
three or four plants). The percentages of hybrids under both condi¬ 
tions were therefore computed for the populations resulting from the 
double-pollination of Pima and of Upland flowers, respectively, and are 
given in Table III. 
Tabi,E III .—Percentages of hybrids in hills containing I or 2 and in hills containing 3 
or 4 plants , respectively , four seeds having been planted in each hill 
Percentages of hybrids in populations resulting from 
double pollination of— 
Number of plants per hill. 
Pima flowers. 
Upland flowers. 
Total 
plants. 
Percentage of 
hybrids. 
Total 
plants. 
Percentage of 
hybrids. 
i or 2. 
676 
1.657 
23. 6±l. IO 
26. 6±o. 73 
3- o±i. 32 
411 
998 
29. 6±i. 52 
26. 3 ±0.94 
3 - 3 ± x - 79 
2 or 4. 
Difference. 
The data presented in Table III show that in the population from 
double-pollinated Pima flowers the percentage of hybrids was greater 
where the conditions were more favorable to survival, while in the popu¬ 
lation from double-pollinated Upland flowers the converse was true. In 
neither population, however, was the difference significant, and it may 
be concluded that the percentages of hybrids obtained in these experi¬ 
ments were not appreciably affected by conditions operating after the 
seeds were planted. 
The evidence seems conclusive that the case under consideration can 
not be accounted for as one of selective survival in the germinating and 
seedling stages. It remains to consider whether selective survival may 
have occurred at the time of fertilization or immediately thereafter. 
Since in the adult population homozygotes were much more numerous 
than heterozygotes, it might be assumed that even if equal numbers of 
the female gametes had been reached by both kinds of male gametes, an 
undue proportion of those reached by the unlike male gametes had per¬ 
ished because fertilization was not completed or because the resulting 
zygotes were unable to develop. Direct evidence on this point would be 
very difficult to obtain but the following considerations make it improbable 
that selective fertilization in this material can be explained in this way. 
The mean number of ovules in the ovary of Pima cotton, as determined 
in 1922, was 21.5 ±0. 10. The mean number of seeds in the bolls which 
