Vol.. 6, 1920 
GENETICS: D. F. JONES 
69 
ence shown by the plant for its own pollen. Since heterosis is roughly 
proportional to germinal diversity, the greater the genetic differences 
there are the greater is the handicap placed upon the pollen when com- 
peting with pollen from the plants on which it is to act. This is a re- 
markable result, because in proportion as the cross-fertilization benefits 
the progeny the less effective are the germ cells in accomplishing fertilization. 
It should be clearly kept in mind that the selective action shown in pollen 
mixtures is not the same as a differential potency of unlike gametes pro- 
duced by one individual. In the latter case there is no positive proof 
that such an effect is ever obtained, where the different gametes are all 
viable, and in most cases there certainly is no such action. 
It has been found that the same selective action is shown when first 
generation hybrids are paired as when strains which have been long inbred 
are used. In other words, the effect is apparent, whether the plants have 
a long line of similar ancestors back of them or whether their immediate 
parents are diverse. It is also shown both in plants of weak growth or of 
full vigor, and whether the gametes of either type or both are alike among 
themselves or are exceedingly diverse in the hereditary factors which they 
carry. The only feature in common is that the cytoplasm is alike for all 
the germ cells of one plant, and this cytoplasm, in self-fertilization, is the 
same in which the pollen fulfils its function. This indicates that the un- 
equal fertilizing ability is governed by the rate of pollen tube growth, 
although it may be determined after the male gametes are brought to the 
egg- 
There is current in biological literature the assumption that hetero- 
geneity in protoplasmic structure is favorable to developmental efficiency. 
This idea has been proposed again and again and applied in many different 
ways. Stated in general terms, it implies that the union of diverse ele- 
ments and the resulting lack of balance stimulates growth. This is a 
heritage from Darwinism, and the writer believes that it is founded upon 
fallacious reasoning and is not supported by the facts. 
The hypothesis has been used in theories of rejuvenation, explanations 
of hybrid vigor, and speculations concerning selective fertilization. The 
necessity for sexual reproduction at some time to maintain organisms 
reproducing asexually is no longer admitted. That the process of forming 
gametes which reunite to make a new individual may bring about a re- 
organization of the protoplasm with elimination of waste products result- 
ing in increased growth is easily conceivable, but the significance of such 
a procedure is not necessarily to be found in the bringing together of unlike 
elements. The vigor of hybridization has now been put on a basis of pure 
-inheritance, and the physiological stimulation hypothesis is no longer 
needed. Homozygous factor combinations, according to present theory, 
are more efficient than heterozygous combinations of the same factors. 
It now seems that self-prepotency, except in those cases where a definite 
