QO BIOLOGICAL LECTURES. 



the "gonidia," with which they live in close symbiotic rela- 

 tions. 1 



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 an 

 independent organism, composed of organs, can 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. 2 

 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 alga nor the fungus would be able to exist separately. 



In dealing with the structures in an organism commonly 

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

 the structure in question is an organ in the physiological and 

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

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

 phological sense, the study of the development of the whole 

 organism 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 proof tJiat tJiey were derived 

 by the differentiation of some common germ ; but, on the con- 

 trary, two dissimilar organisms may, by mutual adaptation, 



1 In using the term " symbiotic," to express the relation between the alga and 

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

 (Die Erscheinung der Symbiose, Strassburg, 1879, P- r 5> et se q-)> Frank (Symbiose, 

 Lehrbiich der Botanik, Bd. I, 1892, Leipzig), and others. 



- See, for example, the recent work by Bonnier, Recherches sur la synthese des 

 lichens, Ann. des sc. nat., 7 me serie, IX, Botanique, 1889. 



