178 THE THEORY OF EVOLUTION 



species enter into close mutual relations, then this is 

 based either on utility on one side only or on mutual 

 support. In the first case we speak of Parasitism, in 

 the second of Symbiosis. 



(1) The adaptive phenomena of the Parasites. 



We will confine ourselves here to the true Parasites, 

 independently of the more or less legitimate kinds of life 

 association which do not imply an actual dependence 

 but rather a companionship. 1 



True Parasites are organisms which exist either 

 upon or in other organisms in order to nourish them- 

 selves by the living substance or already prepared nutri- 

 tive sap of the same. 3 With this peculiar mode of life 

 on or in other living beings (termed ' host animals ' or 

 ' host plants ') the bodily equipment and the needs of 

 the Parasites are in the most perfect accord. The pre- 

 ponderant majority of the Parasites are adapted physio- 

 logically and morphologically to their abnormally 

 deviated mode of existence : physiologically by the fact 

 that they necessarily require for the maintenance of 

 their existence the nutrition to be derived from a definite 

 species of animal or a group of related species ; morpho- 

 logically in so far as their bodily construction is arranged 

 for the acquisition of precisely this nourishment. 3 



1 A short description is found in all botanical and zoological text- 

 books. Prof. L. v. Graff has treated of Das Schmarotzerthum im Tierreich 

 in his work Wissenschaft und Bildung, No. 5, and given an excellent sum- 

 mary of the observations made in that connection. With his interpretation 

 of the facts we cannot, however, always agree. 



2 Ibid. p. 8. 



3 Ibid. p. 9. 



