252 THE SURVIVAL OP THE UNLIKE. [xHL 



rule will be true in those eases in which man desires a 

 lessening of . seed -production in order that some ad- 

 vantage may be gained for the seeds that remain. 



2. Seed-production has decreased, as a rule, in those 

 plants ivhich are propagated exclusively , or nearly so, by 

 separable parts other than seeds. Under this caption 

 may be cited the banana, sweet potato, -potato, horse- 

 radish, sugar cane, some onions, and others. As a rule, 

 all wild plants which propagate readily by tubers, off- 

 shoots, or similar means, produce comparatively few 

 seeds, or, in some instances, none whatever. In this 

 connection it is only necessary to cite the instances 

 of quack grass upon certa,in soils, Canada thistles, 

 and some potamogetons, in support of this proposi- 

 tion. The production of seeds and fruit is an ex- 

 haustive process, demanding much of the plant's vital- 

 ity, and if this vitalit}^ is early diverted into growth 

 of other organs, necessarily the fruit and seeds must 

 suffer. This explains why the early varieties of potatoes 

 produce fewer seeds than the late varieties. The tubers 

 form earlier in the life of the plant, and the plant 

 energy is diverted before the blossoms appear. If the 

 tubers are not allowed to form, the plant produces 

 flowers and fruits more abundantly. This has been 

 proved by Thomas Andrew Knight. For the same 

 reason, flowers on young and thrifty fruit trees do 

 not set fruit, although the flowers may produce good 

 pollen and perfect pistils. Luxuriant growth makes 

 the first demand upon the young tree, and seed -pro- 

 duction suffers. Cultivation lessens seed -production 

 and fruit -production in some of these cjises, simply 

 because it exaggerates the opposing or vegetative 

 methods of propagation through constant breeding 



