THE ORIGINATION OF PARASITISM.* 



The whole tissue of the body of knowledge organized under biological 

 science is interwoven with the conception that organisms in the march of 

 their evolutionary development make purposeful modifications and display 

 direct reactions to external factors, as a result of which they engage more 

 intricately with environmental components and perform the simple and 

 derived functions with greater efficiency. Such teleological ideas lead the 

 van of a host which come forward under the banner of "adaptation," a 

 term so diversely applied as to have become well-nigh useless in critical 

 discussions. This is apparent at once when it is realized that the result 

 of a gradual development of a character in any direction by reason of the 

 selection of minute fortuitous variations may also be termed ' 'adaptations, ' ' 

 especially by those naturalists who depend upon natural selection to ac- 

 count for all evolutionary progress. 



The present discussion concerns the physiologic and morphogenie nature 

 of the alterations which organisms undergo in response to environment, 

 and the whole group of questions as to methods of acquisition, progres- 

 sion, or retrogression fortuitously, purposefully, orthogenetically, contin- 

 uously, or discontinuously, may be held in abeyance for the moment while 

 an attempt is made to show not what living things must do, but what 

 they may do under certain specialized conditions. 



The accommodation of an organism to unusual concentrations of the 

 solutions in the medium or substratum, and the more obvious response to 

 aridity, humidity, and intensities of radiation have been the subject of 

 numerous tests in the laboratory, and carry the general interest that attaches 

 to results which widen the known capacity of organisms with respect to 

 one of the most primitive and perhaps the most essential property of living 

 matter — the power of adjustment to environment. 



The changes constituting these reactions are as inevitable, though not 

 quite so direct, as the melting of ice under heat; they are incidents in 

 autogenetic history and may be reversible in the individual, as water may 

 be frozen again, unless accompanied by morphological maturations. Even 

 when individually fixed, transmission of effects beyond a few generations 

 has not been demonstrated. With reference to their evolutionary value it is 

 to be said that such accommodations may be inhibitive to important func- 

 tions and deleterious if preserved, while, on the other hand, it is obvious 



♦The principal conclusions embodied in this paper were discussed in part before the 

 Sigma Xi Society of the University of Chicago, December 9. L909; the Scientific Soci- 

 ety of Johns Hopkins University, December 20, 1009; and the American Naturalists, 

 Boston, December 22, 1000. 



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