2o6 Transactions British Mycological Society. 



(i) The characters separating one species from another are 

 of morphological natm*e. 



(2) The essential specific characters are constant and here- 

 ditary ("nulla certior. . .quam distincta propagatio ex semine"). 



(3) The essential specific characters of an organism may be 

 determined and evaluated by sight in one specimen of one 

 generation. 



Now these assumptions, and all that gigantic and rather 

 wonderful superstructure of fact and fancy which is erected 

 upon them and which we call systematic mycology, take origin 

 in a philosophic concept, that of the species. The essential 

 quality of a species is that the different individuals included 

 in it be identical, and throughout all systematic mycology 

 morphological comparison is accepted as a criterion for specific 

 purity. The species concept therefore may perhaps be expressed 

 as follows — a species consists of the total of individuals possessing 

 essentially similar morphological facies. The species is thus a 

 morphological abstraction. 



Such a species concept is not of course confined to mycology 

 but is common to and is held with varying degrees of rigidity 

 in all branches of systematic biology. 



So far as the flowering plants are concerned, however, it has 

 long been recognised that the morphological facies of an 

 organism may vary according to development under different 

 environmental conditions. For example Leontopodiiim alpinum 

 growing on a mountain summit is hardly recognisable as the 

 same species as this plant growing in a lowland meadow. 

 Polygonum amphibium has a terrestrial variant extremely 

 different from the aquatic or the xerophytic growth form; 

 and so also Raiutnctdus sceleratus and many other plants. 

 These organisms are plastic and the particular morphological 

 facies of a specimen is clearly a resultant of two systems of 

 interacting forces represented by the living substance of the 

 organism and the conditions of life*. In the phanerogams, 

 however, it is but rarely that these ecologic growth forms or 

 "ecads," to use the name given to them by Clements(6), differ 

 markedly in the essential morphological characters of their 

 reproductive members, and consequently the underlying specific 

 identity may usually be traced with ease throughout. 



* As Baur says (Einfiihrung in die Exakte Vererbungslehre Aiif. 2, 1914) 

 " Only a certain specific type of reaction to the external world is inherited, and 

 what we perceive with our senses as external characteristics are only the result 

 of this reaction on a chance combination of external conditions under which 

 the individual has developed." And MacLeod (24, 2), "The so-called character- 

 istics of each species are the product of reactions, in which there intervene on 

 the one hand the external causes which affect the individuals during their 

 development, and on the other hand the living substance of the species under 

 consideration." 



