ASPECTS OF KINETIC EVOLUTION 38 1 



by all breeders. That the same principles apply to plants, has 

 remained in doubt for two reasons : (1) The much less com- 

 plex organization and less specialized tissues of plants render 

 many of them less acutely dependent upon cross-fertilization. 

 (2) The plants which have been longest under cultivation are 

 not grown for their seeds and are propagated asexually, so 

 that their decline in reproductive fertility has not diminished 

 their economic value. No plant valued for its seeds has been 

 propagated other than from seeds for any considerable period. 1 

 Numerous tropical root-crops and fruits, such as the sweet-po- 

 tato, yam, agave, sugar-cane, banana, pine-apple, and bread- 

 fruit have been grown for thousands of years from cuttings, prob- 

 ably without the interposition of a single seedling generation. 

 In a sexually propagated species inbreeding would have led 

 long since to extinction, but these clonic varieties are still ex- 

 tremely vigorous. Nevertheless, such species do not form a 

 real exception to the rule of deterioration under inbreeding, since 

 a very large proportion of them, belonging to many and very 

 diverse families, have shown this tendency towards seedlessness. 

 The reduction or elimination of the reproductive parts has 

 been ascribed by some to selection, and by others to a supposed 

 biological law of paucity which causes useless parts to disap- 

 pear. No basis of fact has been shown, however, for either of 

 these explanations ; unassisted nature supplies us with instances 

 like Sphagnum and Lunularia to which neither would logic- 

 ally apply, but which would be well accommodated in the 

 view that continued asexual propagation, like other forms of 

 isolation, weakens the reproductive powers. This law would 

 also explain why the absence of sexual reproduction ap- 

 pears only as the character of aberrant species or genera, and 

 has not been able to persist for a period long enough to permit 

 the differentiation of organic groups of higher systematic rank. 

 Botanists seem not to have ascertained the existence of any wild 

 phanerogamous plant which is always and everywhere seedless. 



'Apparent exceptions to this rule appear only among trees, such as the 

 almond and the pistache, where the normal long life of the individual may be 

 thought of as lessening the period of vegetative propagation, if counted by 

 generations. 



