Chap. XVIII. STERILITY. 155 



cases analogous with the foregoing could be given; for instance, 

 some kinds of mosses and lichens have never been seen to fructify in 

 Prance. 



Some of these endemic and naturalised plants are probably 

 rendered sterile from excessive multiplication by buds, and their 

 consequent incapacity to produce and nourish seed. But tho 

 sterility of others more probably depends on the peculiar condi- 

 tions under which they live, as in the case of the ivy in the northern 

 parts of Europe, and of the trees in the swamps of the United 

 States; yet these plants must be in some respects eminently well 

 adapted for the stations which they occupy, for they hold their 

 places against a host of competitors. 



Finally, the high degree of sterility which often accom- 

 panies the doubling of flowers, or an excessive development of 

 fruit, seldom supervenes at once. An incipient tendency is 

 observed, and continued selection completes the result. The 

 view which seems the most probable, and wdiich connects 

 together all the foregoing facts and brings them within our 

 present subject, is, that changed and unnatural conditions of 

 life first give a tendency to sterility; and in consequence of 

 this, the organs of reproduction being no longer able fully to 

 perform their proper functions, a supply of organised matter, 

 not required for the development of the seed, flows either into 

 these organs and renders them foliaceous, or into the fruit, 

 stems, tubers, &c, increasing their size and succulency. But 

 it is probable that there exists, independently of any incipient 

 sterility, an antagonism between the two forms of repro- 

 duction, namely, by seed and buds, when either is carried to 

 an extreme degree. That incipient sterility plays an impor- 

 tant part in the doubling of flowers, and in the other cases 

 just specified, I infer chiefly from the following facts. When 

 fertility is lost from a wholly different cause, namely, from 

 hybridism, there is a strong tendency, as Gartner 118 affirms, 



70;Vaucher, 'Hist. Phys. Plantes phar,'" Abhand. Naturw. Gesellsch. zu 



d'Europe,' torn. i. p. 33 ; Lecoq, ' Geo- Halle, 5 B. xi. 1870, p. 40, 78. 



graph. Bot. d'Europe,' torn. iv. p. 118 ' Bastarderzeugung,' s. 50S 



466 ; Dr. D. Clos, in ' Annal. des Sc. Kolreuter (Dritte Fortsetzung, s. 



Nat.,' 3rd series, Bot., torn, xvii., 73, 87, 119) also shows that wher. 



1852, p. 129: this latter author refers two species, one single and the other 



to other analogous cases. See more double, are crossed, the hybrids are 



especially on this plant, and on other apt to be extremely double, 

 allied cases, Prof. Caspary, " Die Nu- 



