INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION 263 



exercise any influence upon the flowers and seed-forming of this 

 species ? In point of fact it has exercised none in most potatoes, for 

 the flowers and seeds are just as fertile now as they were when the 

 potato was flrst discovered. 



Whether the pollen of a flower is utilized in one or other of its 

 thousands of pollen-grains by reaching the stigma of another plant of 

 the same species, or whether all the pollen-grains are uselessly scattered 

 abroad, cannot possibly aff'ect the flower so as to cause degeneration ; 

 the theory of disuse cannot be applied in this case. What is true of 

 the potato holds good also of the manioc (Manihot utiUssima), but, 

 on the other hand, many of the best varieties of common fruits — 

 pears, figs, grapes, pine-apples, and bananas — are seedless. In Maranta 

 arundinacea ' the whole wonderful structure of the flower has per- 

 sisted, but the pollen-grains, that is the germ-cells, are wanting.' 

 Whether this implies a permanent degeneration of the sexual organs, 

 that is to say, one that is embodied in the primary constituents of the 

 species, or whether it is only the result of over-abundant nourishment, 

 or of other causes in the circumstances affecting the particular plant, 

 can only be decided by experiment. Probably both occur. The 

 common ivy, for instance, does not now blossom in the northern parts 

 of Sweden and Russia, but it does so still in the southern provinces. 

 If plants were brought to us from the most northerly zone of distribu- 

 tion, they would in all probability flower and bear fruit with us, and 

 in that case the absence of bloom in these plants must have been 

 a direct effect of the cold climate. But it is quite conceivable that 

 cultivated plants have in many cases become hereditarily infertile, 

 when they are constantly propagated only by means of buds, layering, 

 and so on, not however because of any direct effect of this mode of 

 propagation, but through chance germinal variations. For in regard 

 to many of them man has lost all interest in the flowers and fruit, 

 as, for instance, in the case of the potato ; in other cases he is even 

 interested in procuring seedless fruits. 



In the first case he will quite readily make use of plants with 

 imperfect flowers for propagating, if they are otherwise fit and exhibit 

 what he wants in other respects; in the second case, he will give 

 a preference to individuals with seedless fruits, and thus increase and 

 strengthen the tendency to degeneration of the seeds in the race 



concerned. 



All these cases are quite in harmony with our conception of 

 amphimixis, which, now that we have investigated the facts through- 

 out the animate kingdom, we may sum up in the following propositions. 

 In the whole orp-anic world, from unicellular organisms up to the 



