246 COMPENDIUM OF GENERAL BOTANY. 



nor to external causes (influence of nutrition, light, temperature,, 

 mechanical stimuli, etc.). Nageli's theory also differs from em- 

 piricism in regard to what external and internal causes can pro- 

 duce. In one important point we agree with Nageli; that is, as to 

 the influence of nutrition. This factor is plainly under the control 

 of direct observation. 



' ' No inherited property, no variety, race, or species, owes its 

 origin to nutritive processes." It is important to bear in mind 

 that the same varieties may occur in localities widely different, and 

 that two slightly different varieties may occur in the same locality. 

 Years ago Nageli also pointed out the following general phenome- 

 non. Mountain-plants transplanted to the valley lose their moun- 

 tain habits, although they evidently lived among the mountains for 

 thousands of years. Climatic influence therefore does not produce 

 constancy. 



Nageli assumes that the " stimuli " 1 resulting from internal 

 and external causes are active in producing new species, genera, etc. 

 The internal causes are supposed to lie in the hidden nature and 

 structure of idioplasm. These causes produce a continual progres- 

 sive change in the micellar structure of the idioplasm, causing it to 

 become more and more complicated and highly organized (principle 

 of perfecting); primarily, all hereditary transformations due to ex- 

 ternal causes are the result of idioplasmic changes. Progressive 

 organization and division of labor are in general induced by inter- 

 nal causes ; while the specific constitution and variation in form, 

 organization, and division of labor, the adaptation to the external 

 environment, are the result of external causes (stimuli). Nageli's 

 theory differs from that of DARWIN. According to the latter, the 

 external causes act negatively in repressing or stamping out that 

 which is not suited or adaptable (natural selection with struggle for 

 existence). According to Nageli, the external stimuli act mechani- 

 cally upon the micellar structure of the idioplasm" to produce the 



1 By " stimuli" Nageli means those influences upon the plant-organism which 

 induce reactionary effects, as mechanical stimuli, light, warmth, cold. In 

 Nageli's Abstammungslehre, p. 102, there occurs a contradictory statement : most 

 climatic influences are classified with the indifferent influences, while on the above 

 page warmth and cold are classed among the effective stimuli which aid in form- 

 ing hereditary qualities. 



2 Abstammungslehre, p. 139. 



