2 



go BIOLOGICAL LECTURES. 



the " <;-()nidia,"' with which they live in ch)se symbiotic rela- 

 tions.^ 



Thus the morphological and physiological consideration of 

 an organ, such as we have just given, leads us to conclude that 

 when we find two structures in an individual organism most 

 intimately associated in their physiological relationship, it does 

 not necessarily follow that they are organs in the morphological 

 sense also. The history of the lichen clearly shows that cm 

 iiidcpcndcut organism, composed of orgcuis, coin be created by tJie 

 union of two dissimilar organisms by the establishment of an 

 intimate physiological relationship between them. In fact, a 

 certain number of species of lichens have been actually pro 

 duced by bringing fungi and algae together in a synthetic way 

 The fungus and alga, by the interchange of their metabolic 

 products, supply the nutritive wants of each, and thus produce 

 the autonomous whole which can exist in places where neither 

 the alija nor the fungus would be able to e.xist separately. 



In dealing with the structures in an organism commonly 

 called organs, I repeat, we must clearly bear in mind wdiether 

 tl-ie structure in question is an organ in the physiological and 

 morphological senses, or whether it is an organ simply in the 

 physiological sense. If the structure is an organ in the mor- 

 phological sense, the study of the development of the whole 

 or<^anism will show that it is a part of the products of differ- 

 entiation of some preexisting germ from which the entire 

 organism was derived ; if it be an organ in a purely physio- 

 logical sense alone, there will be no genetic connection between 

 the different structures, although each is indispensable to the 

 existence of the other. In short, the perfection to which the 

 physiological adaptation of different organs is carried out in a 

 given organism, is, in itself, no procf that they were derived 

 by the differentiation cf some common germ : but, on the con- 

 trary, t7U0 dissimilar organisms may, by mutual adaptation, 



1 In u.sing the term "symbiotic," to express the relation between the alga and 

 fungus in the organism of a lichen, I simply follow such botanists as JJe Bary 

 {Die Ersckeinung der Sytnbiose, Strassburg, 1879, P- '5- *^t seq.), Frank {Symbiose, 

 Lehrbuch der Botanik, Bd. I, 1892. T,eipzig), and others. 



- See, for example, the recent work by Bonnier, l\ccln-rihes sttr la syiitJiisc dcs 

 lichens. Ann. des sc. nat., 7'"^ serie, IX, Botanique, 18S9. 



