18G TJtc Unitij of the Organism 



of the members to the partnership actually enters the other 

 and lives upon it at some stage of the game, so the "living 

 tcgetlier" Is really a sort of parasitism. The taxonomic 

 identitv of the "consortia," as the partners are sometimes 

 called, is lost. Concerning the most famous symbiosis known, 

 tliat of tlie lichens, a class of plants which owes its very 

 existence as a botanical group to the intimate association 

 that iias been contracted between plants of two other such 

 groups, fungi and algae, we read, "Strictly speaking, both 

 fungi and algae should be classified in their respective orders; 

 but the lichens exhibit among themselves such an agreement 

 in their structure and mode of life, and have been so 

 evolved as consortia, that it is more convenient to treat 

 them as a separate class. . . . From the symbiosis entered 

 into by a lichen fungus with an alga, a dual organism re- 

 sults with a distinctive thallus, of which the form (influenced 

 by the mode of nutrition of the independently assimilating 

 alga) differs greatly from that of other non-symbiotic 

 Eumycetes." ^ 



As a purel}'' imaginar}^ construction one might, perhaps, 

 picture an organism produced in this fashion which would 

 not be "dual," as these authors express it, but monal, that 

 is, an organism in the usual zoological and botanical sense. 

 But since such an organism would be a work of the imagina- 

 tion, pure and simple, with all observational evidence weigh- 

 ing against its real existence, this particular form of the ag- 

 gregational conception of the organism can not be held to 

 have any scientific value, especially" if the aggregants or. con- 

 sortia be imagined to be cells. 



(c) As tested bij the Specificity ajid Metaplasy of 



Differentiated Cells 



The question now before us is, how far are cells which are 

 wliolly or largely differentiated into tissues bound, willy nilly, 

 to continue to be just those tissues, and to produce as they 



