50 INVASION 



intensity and duration of the agent, as well as by the 

 character of the space through which the latter operates. 



ECESIS. 



By the term eoesis is designated the series of phenomena 

 exhibited by an invading disseminule, from the time it 

 enters a new formation until it becomes thoroughly estab- 

 lished there In a word, ecesis is the adjustment of a plant 

 to a new habitat. It comprises the whole process covered 

 more or less incompletely by acclimatisation, naturalisation, 

 accomodation, etc. It is the decisive factor in invasion, in 

 as much as migration is entirely ineffective without it, and is 

 of great value in indicating the presence and direction of 

 migration in a great dumber of species where the dissem- 

 inule is too minute to be detected, or too little specialised to 

 be recognisable. 



Homboldt (1805:21) touches the subject of ecesis only in- 

 cidentally in speaking of the lack of sufficient flexibility in 

 any known species of phanerogams to enable it to accomo- 

 date itself to all latitudes and altitudes. DcCandoIIe {iS20- 

 5t) deals with naturalisation in a very general way, and 

 cites several instances of its operation in adventive species. 

 Henfrey (1852:51) speaks of the inability of alpine plants to 

 accomodate themselves completely to lowland conditions, in 

 which, however, they often persist in consequence of con- 

 tinuous migration. A. DeCandoIIe (J855:623) considers 

 naturalisation at length, but almost wholly with reference 

 to results, so that his excellent discussion belongs almost 

 entirely to distribution. He gives a comprehensive though 

 summary account of the obstacles to naturalisation, treats of 

 the latter at small and great distances, and concludes with 

 an account of the origin of cultivated species. Darwin 

 (1859), in his chapter on the struggle for existence, discusses 

 with more or less detail the biological conditions under 

 which ecesis takes place in an invaded formation. He 

 argues for a wide range of adaptability in most organisms, 

 on account of their power of acclimatisation, though he does 

 not produce any evidence of this from the plant kingdom. 

 Gfisebach (1872:215, 264) pays especial attention to the inter- 



