ECESIS 51 



change of arctic and alpine plants, and formulates the funda- 

 mental law of barriers, viz.,: "les plantes endemiques 

 augmentent en raison des obstacles qui s'oppoaent h leur ex- 

 tension." Nageli (1874:109) lays down an exact procedure 

 for investigating the struggle for existence. His most im- 

 portant conclusion is that a stronger plant rarely drives the 

 weaker one out entirely, but that their numerical relation is 

 an expression of the ratio between their capabilities for 

 existence. Engler (1879: XI) recognizes that the viability of 

 seeds differs widely, and that the chances of ecesis depend 

 directly upon the length of time necessary for migration. 

 He connects ecesis directly with distribution in the principle 

 that "Allmalig verbreiten sich die Pormen eines Pormen- 

 kreises soweit Bodenverhaltnisse, klimatische Verhaltnisse, 

 iind Concurrenz anderer Planzen es gestatten. " Watming 

 (1895:351), in connection with commensalism and succession, 

 summarizes in a comprehensive manner the effect of occupa- 

 tion upon ecesis. Schimper (1898:55) treats acclimatisation 

 briefly, taking into consideration adjustment to temperature 

 alone. 



The relation of migration to ecesis is a most intimate 

 one: the latter depends in a large measure upon the time, 

 direction, rapidity, distance and amount of migration. In 

 addition, there is an essential alternation between the two, 

 in as much as migration is followed by ecesis, and the latter 

 then establishes a new centre from which further migration 

 is possible, and so on. The time of year in which fruits 

 mature and distributive agents act has a marked influence 

 upon the establishment of a species. Disseminules designed 

 to pass through a resting period are often brought into con- 

 ditions where they germinate at once, and in which they 

 perish because of unfavorable physical factors, or because 

 competing species are too far advanced. On the other hand, 

 spores and propagules designed for immediate germination 

 may be scattered abroad at a time when conditions make 

 growth impossible. The direction of movement is decisive 

 in that the seed or spore is carried into a habitat sufficiently 

 like that of the parent to secure establishment, or into one 



