THE ORIGINATION OF PARASITISM. 59 



quite reasonable that new parasitic unions are being eonstantly formed in 

 almost all kinds of habitats. This fact might escape detection by ordinary 

 methods of observation almost indefinitely. 



The results of this and previous studies allows us to recognize the limit- 

 ing- factors, the operative conditions, and some of the facts as to the physio- 

 logical nature of parasitism. Few specializations present their main 

 features so clearly to the student. When the total body of evidence, how- 

 ever, is evaluated in the light of current theories, it is found that it is no 

 easy matter to decide by what main methods of evolutionary procedure 

 this adjustment or "adaptation" is attained and how it is advanced from 

 stage to stage. 



Some alterations, such as that of Passifiora on Euonymus, are distinctly 

 discontinuous, but in this particular instance it can not be shown that any 

 permanent results would follow the occurrence of the dependent relation. 

 Some features, however, such as the development of haustoria on roots 

 and stems, would be a distinct mutation and is well illustrated by Krameria 

 and supposedly also by Cuscuta. The taking on of absorptive functions 

 by the epidermal cells of xeno-parasites as described in the present paper 

 is to be recognized as a distinct mutation. 



A direct and immediate atrophy of the various organs of the shoot 

 ensues as a result of the assumption of the parasitic relation, but the ex- 

 treme stages of such reductions appear to have been reached by gradual 

 changes, although it is evident that such a conclusion is almost wholly 

 inferential. The transition from autophytism to complete parasitism, with 

 attendant habital characters, appears to have been gradual, since species may 

 be cited to illustrate all degrees of the alteration; but on the other hand it is 

 not impossible that the complete change may have been made at once in 

 some forms. Nothing in the entire matter suggests progression or retro- 

 gression of all of the involved characters by one method alone. 



Viewed from another angle, it may be seen that some of the alterations 

 described may be taken to be directly interlocking, or reactive, essential, 

 and practically irreversible. Correlated with these, having no direct con- 

 nection with originating external causes or limiting conditions, but inev- 

 itably consequent upon the primary alterations, are a number of secondary 

 changes which may be the most obvious, but in reality of lesser importance. 

 Modifications of the absorbent organs and of the nutritive systems would 

 be included in the first, while incidental atrophies and other characters, such 

 as the striking nutatory movements described, would be examples of the 

 second. The ingenious and intricate and strained interpretations made to 

 include all of the phenomena displayed by organisms which bear special- 

 ized relations to other organisms or to environmental factors are in strik- 

 ing contrast with this view. 



A good mechanical illustration of the alterations of an organism, in its 

 adaptive adjustment to any factor, is offered by the behavior of a drop of 



