328 Lvolution and Adaptation 
of a naked drop of plasma with or without amembrane. But, 
according to the further Darwinian conception, competition, 
leading to extermination, is capable of bringing such a condi- 
tion toa 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 forms 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 
