ON GENERA AND SPECIES. 3 



but the different views entertained by botanists, respecting 

 the data on which genera and species are founded, renders 

 the study of Perns a task of no ordinary labour. This is 

 owing to many causes, one of which is found in the fact 

 that many species are normally heteromorphous ; separate^ 

 plants of the same species presenting different forms at 

 the same or at different periods of growth, although grow- 

 ing under the same climatic influences ; in some cases dif- 

 ferent fronds of the same plant, and even portions of the 

 same frond when seen apart, have been characterized as 

 distinct species, and have even been placed under different 

 genera. It also frequently happens that two or more speci- 

 mens present suiEcient characters to be considered distinct 

 species, but when the intermediate gradations of the 

 series are supplied, only the most extreme states of &ny 

 of the forms appear sufSciently distinct to be so con- 

 sidered. It often happens that plants of the same species 

 at one time produce all simple, and at another time all 

 pinnate or more highly divided fronds ; it is therefore not 

 surprising that different forms of individual species should 

 be viewed as representing two or more species when 

 known only from incomplete herbarium specimens. 



It must also be borne in mind that the descriptions of 

 many species found in books are derived from dried speci- 

 mens, obtained in various parts of the world by trade 

 collectors whose object is often only pecuniary gain, and 

 whose profits are dependent on the number of forms found. 

 Great caution is therefore necessary in accepting a collector's 

 idea of species ; it is only on the woi'd of the independent 

 and truly scientific collector that we can rely for correct 

 information on the various aspects assumed by individual 

 species growing- in regions far away from where we write 

 their history ; bat even under the most favourable circum- 



