REPRODUCTION AND DISPERSAL 867 



problem of cross pollination is essentially a phase of the problem of 

 sexuality, which has been considered previously. 



Experimental data. — ■ It long has been believed by plant breeders 

 that occasionally crossing is necessary if the individuals of a species 

 are to be kept in a state of the highest vigor, inbreeding {i.e. breeding 

 between closely related forms, as in close polHnation) often having been 

 shown to result in diminished luxuriance in the progeny. Careful ex- 

 periments conducted many years ago on a number of species, though 

 yielding rather discordant results, tended to show that cross polHnation, 

 on the whole, is more advantageous than close pollination. In some 

 cases diminished vigor, which is made evident by smaller size and les- 

 sened seed production, is obvious in the progeny of the first generation (as 

 in Ipomoea purpurea and Mimulus luteus) ; in a much larger number of 

 cases it is obvious only after continued inbreeding for several genera- 

 tions, and in still other cases inbreeding appears never to result in 

 deterioration. 



Recent experiments made on Indian corn show that close pollination 

 results in the first generation in reduced height and yield; in the gener- 

 ations following there is further reduction, but the decrement becomes 

 less each time until about the fifth generation, after which continued 

 close pollination makes no appreciable change in the offspring. Such 

 dwarfs may reproduce as such indefinitely, but if crossed the offspring 

 of the first generation has the original full size. In the earlier experiments 

 geitonogamy usually resulted in quite as much weakening as did autog- 

 amy, showing conclusively that it is much more nearly related to close 

 pollination than to cross pollination. Progeny from crossed individuals 

 were found to flower first and to suffer less in crowded cultures than 

 did progeny from inbred races; this fact has been taken to be of great 

 significance, since it suggests the likeKhood of the submergence of inbred 

 races by cross-pollinated races in the " struggle for existence " in nature. 



The possible advantages of cross pollination. — The experiments cited 

 may mean either that cross pollination is in some way advantageous, 

 or that close pollination is in some way disadvantageous. The support- 

 ers of the first theory have held that cross pollination is advantageous, 

 because it insures the mingling of two lines of ancestry with their sup- 

 posedly different beneficial characters, or because it promotes varia- 

 bility or racial vigor. There is very little in favor of this view and very 

 much against it. In the first place, experiments on the same species 

 at different times and places vary widely in their results; sometimes own 



