328 Evolution and Adaptation 



of a naked drop of plasma with or without a membrane. But, 

 according to the further Darwinian conception, competition, 

 leading to extermination, is capable of bringing such a condi- 

 tion to a higher stage of development, since it is assumed that 

 those individuals which vary in a beneficial direction would 

 have an advantage over those that have not taken such a 

 step, or have made a step backward. 



If, on the other hand, under the above-mentioned conditions 

 of unrestricted development, without competition, variations 

 were determined by " mechanical principles ," then, according 

 to Nageli's view, all plant foi'ms that now exist would still 

 have evolved, and would be found living at the present time, 

 but along with all those that now exist there would be still 

 other forms in countless numbers. These would represent 

 those forms which have been suppressed. On Nageli's view 

 competition and suppression do not produce new forms, but 

 only weed out the intermediate forms. He says without com- 

 petition the plant kingdom would be like the Milky Way ; in 

 consequence of competition the plant kingdom is like the 

 firmament studded with bright stars. 



The plant kingdom may also be compared to a branched 

 tree, the ends of whose branches represent living species. 

 This tree has an inordinate power of growth, and if left to it- 

 self it would produce an impenetrable tangle of interwoven 

 branches. The gardener prevents this crowding by cutting 

 away some of the parts, and thus gives to the tree distinct 

 branches and twigs. The tree would be the same without the 

 watchful trimming of the gardener, but without definite form. 



Nageli states : " From my earlier researches I believe that 

 the external influences are small in comparison to the internal 

 ones. I shall speak here only of the influences of climate and 

 of food, which are generally described as the causes of change, 

 without however any one's having really determined whether 

 or not a definite result can be brought about by these factors. 

 Later I shall speak of a special class of external influences 



