DR. THEODOR NEUMANN. 207 



tain an easy existence, cling to others, adapt themselves 

 to all the qualities and even whims of their hosts, and 

 have the latter provide entirely for their food and lodg- 

 ing? To-day v^hen the life of these parasites lies before 

 us clear and distinct, we know that they indeed ar- 

 range their own existence according to sach motives ; 

 and just as their morals (if there be any ! ) are none too 

 praiseworthy, their physical development is also very 

 much neglected. 



It is a well known fact that all organisms, as to their 

 form and development, are dependent on exterior condi- 

 tions of existence, to which they must adapt themselves. 

 The forms of the animal body, its inner and outer con- 

 struction, have been developed in the course of time 

 through such adaptation ; and they change even now 

 with changing environment. But the kind of develop- 

 ment which parasites were and are still undergoing 

 is entirely different from that of other organisms. 

 The latter showed a progressive character; they became 

 more complicated in the course of time, in proportion as 

 their surroundings lost theii- former simple character; the 

 parasites, on the contrary, show us how certain conditions, 

 which make life especially easy, by providing the neces- 

 sities of existence without hard labor, may cause degen- 

 eration in an organism which was at first highly de- 

 veloped and of complicated structure. All parasites 

 show such degenerating effects of their mode of living; 

 some more, others less, according to the round which 

 they have reached on the ladder of parasitism. Before 

 considering the circumstances, it will be necessary to in- 

 quire what forms and beings must be included in the 

 name " parasites." 



We shall soon see that a strict definition of the word 

 and the idea -'parasite" is difficult, if not impossible; 

 for parasitic life is developed by degrees from ordinary 

 life, and a definite boundary line between parasites and 



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