OTHER WAYS OF PROCURING FOOD 67 



at least a partial or serazparasite. Such plants, which make a 

 part of their food and get the rest from other sources, are also 

 said to be mixotrophic. To this same group belong several members 

 of the figwort family, e. g., Euphrasia, Pedicularis, and Odontites. 



Gradations.— Between complete and partial parasitism and 

 between parasitism and mutualism are to be found all possible 

 gradations. It is often difficult to say just how much advantage 

 each member of the partnership is reaping from the relationship, 

 so that there are all manner of intermediate stages possible between 

 complete parasitism on one hand and perfect mutualism on the 

 other, with partial parasitism as one of the intermediate steps. 

 Helotism (slavery) is such an intermediate condition in which one 

 member of the firm seems to profit while the other continues its 

 existence with neither appreciable gain nor loss. Helotism is 

 more common among the lower plants than in the Spermatophytes, 

 but if the chloroplasts were captured one-celled organisms, as has 

 been suggested (Chap. V), this would be a helotic relationship. 



Man and his domestic animals furnish analogies in the animal 

 kingdom of these same relationships which illustrate the difficulty 

 of determining exactly where mutualism stops and parasitism 

 begins. Is man's relation to horses mutualistic or parasitic? Does 

 a cow profit more or less than a horse from her human relationships? 

 In fact, man's relation to woman has gone through many of these 

 stages. Among barbaric peoples the wife was a slave who was de- 

 pendent for her very existence upon the caprices of her husband 

 and master. This was pure helotism. Gradually the position of 

 woman rose among civilized peoples until the twentieth century, 

 when the marriage relationship had reached the phase of mutual- 

 ism, each partner profiting equally. In these modern days, how- 

 ever, when woman has taken over many of the privileges of man 

 without showing a willingness to assume at the same time his 

 responsibilities, marriage seems commonly to have become a form 

 of parasitism. Continuing this analogy, saprophytism is living 

 from life insurance. 



Facultative and Obligate Saprophytism and Parasitism.— 

 Some organisms can live either heterotrophically or independently, 

 that is, they have the power to make their own food but may, if 

 conditions are suitable, also get nourishment parasitically from 

 living organisms or as a saprophyte from dead organic matter. 

 A plant which can live only as a saprophyte is said to be an obli- 



