868 ECOLOGY 



pollen may be quite impotent, sometimes imperfectly potent, and some- 

 times as potent as foreign pollen. Usually these variations are due to 

 external factors, such as differences in moisture, light, etc. Many of 

 the very species that have been supposed to demonstrate the benefits of 

 cross pollination grow under certain conditions quite as well when 

 close pollinated. Similarly, the results of cross pollination vary widely 

 under different external conditions. In case of crossing between un- 

 related individuals of inbred ancestry grown under similar external con- 

 ditions, the progeny if grown under like conditions are scarcely more 

 vigorous than with close pollination, showing that the vigor supposed 

 to result from crossing may result rather from favorable external con- 

 ditions. Comparable phenomena are known also in the animal king- 

 dom. 



The data from heterostyled plants oppose the usual theory of rross 

 pollination, since weak individuals or none at all result from crossing be- 

 tween plants with styles of unequal length, irrespective of nearne;s or 

 remoteness of relationship; in such cases the progeny is weak, regar Hess 

 of the mingling of " diverse racial characters." Dichogamy is cit< d as 

 affording a priori evidence that cross pollination must be beneficial , yet 

 dioecious plants (in which close pollination is of course impossible) ex- 

 hibit the earlier maturation of anthers or of stigmas almost as habitually 

 as do monoclinous plants. 



The chief reasons for disbelieving that occasional cross pollination is 

 necessary in order to prevent the deterioration of plant species are 

 afforded by the phenomena of autogamy and vegetative reproduction. 

 Usually the argument for the value of cross pollination is built on those 

 cases in which foreign pollen is prepotent or own pollen impotent, omit- 

 ting the equally numerous and significant cases where own pollen is fully 

 potent. Apart from the vast number of plants that exhibit frequent 

 autogamy, there are many in which autogamy is habitual, particularly 

 the numerous species with cleistogamous flowers; in these there is no 

 deterioration, in spite of repeated close pollination. In the experiments 

 cited above, autogamy in several forms (as in Petunia, Eschschcltzia, 

 and Nicotiana) resulted in progeny that was essentially as vigorous 

 as when xenogamy was employed. Even in the two species (viz. Ipemoea 

 purpurea and Mimulus luteus) which were thought most clearly to 

 demonstrate the benefits of cross pollination, luxuriant sports arose in the 

 autogamous cultures, which had as vigorous progeny when close polli- 

 nated, as did cultures from cross-pollinated individuals. Perhaps most 



