182 ECOLOGY 



organic and the inorganic worlds, were advancing and the vast 

 amount of material already collected was being worked over, a special 

 branch of science had arisen, namely, the biology of the flower, which, 

 instigated by Darwin's work upon the mutual relations existing 

 between different organisms, set aside the ancient opinion that 

 zoology and botany could proceed side by side with absolute dis- 

 regard of their dependence upon one another. 



The pivot upon which investigations concerning the pollination 

 of flowering plants turned was the "law of avoidance of self-fertil- 

 ization." Facts which to-day are universally taught in the schools 

 still remained to be proved by such men as Darwin, Hildebrandt, 

 Hermann Miiller, and others. It was discovered with astonishment 

 that Koelreuter toward the middle of the eighteenth century, and K. 

 Sprengel in 1793, had already disclosed "the secret of Nature in the 

 structure and fertilization of flowers." Until then, however, no one 

 had applied the biological relation between flowers, wind, and the 

 insect world, as shown in the phenomena of pollination, important 

 enough to be included in the science of botany, nor had any one 

 made clear the mutual dependence of the animal and plant king- 

 doms in their household economy. These factors in the struggle 

 for existence were now given their full value, and the different appear- 

 ance of various associations was explained by the absence of this or 

 that insect, while in tracing the boundaries which limit the area of dis- 

 tribution of certain plants and animals, as, for instance, Aconitium and 

 Bombus, the organisms were found by Kronfeld to be interdependent. 



Thus in the fourth period of the development of ecology, widely 

 separated branches of science are brought together in order to explain 

 the life-history of a certain region. These, however, must be united 

 in the realm of geography to form a new entity, and, by their correla- 

 tion of numerous data, prove the propriety of their use. The evolu- 

 tionary tendency, which extends far back into the geological past, 

 and is based upon knowledge of the areas of distribution of hosts 

 of related genera and species, has been considered, especially in 

 botanical geography, as a subject quite separate from that which 

 treats of the vegetative forms and the formations as physiological 

 entities, dependent upon external factors. 



The division of entire continents into zones, as seen in the atlases 

 of physical geography, gave expression to this feeling. It was also 

 necessary to extend the ecological method to the field covered by the 

 publication of special floras, and with renewed interest and zeal to 

 begin the revision of the enormous mass of material collected 

 therein. This work had already been begun in the floras of central 

 Europe, which were soon followed by the excellent departmental 

 work of the North American Surveys. Occasional brilliant descrip- 

 tions of tropical floras, such as the descriptions of the vegetation 



