40 The Unity of the Organism 



measure an adaptation to one another and have some struc- 

 tural dependence upon and correlation with one another, 

 that it would be superfluous to enumerate the facts and di- 

 late on their significance. The subject constitutes no small 

 part of the older comparative anatomy and physiology. 



Almost as obvious is it, too, that the major parts of such 

 animals are incapable of long-continued life when they are 

 severed from the whole. But the great capacity for con- 

 tinuance in the living state possessed by certain parts of 

 some classes of animals has attracted much attention, most- 

 ly because of the intrinsic physiological and morphological 

 importance of the phenomena themselves rather than of any 

 assumed support afforded by them to the doctrine of au- 

 tonomy of the parts in a strictly elementalistic sense. But 

 these and other facts of organ-independence have been used 

 as a groundwoi^k for certain elementalistic conceptions of 

 the organism which, viewed in their liistorical setting, are of 

 much broader interest. The historical setting to which I 

 refer goes back to a speculation by that primal elementalist 

 Empedocles, and may be called an organ-assembling theory. 

 The modern relatives of this old theory may be called ag- 

 gregational theories, and are typified by the conception that 

 the normal individual plant or animal is an affair of sym- 

 biosis or secondary union of previously independent organ- 

 isms. A concise statement of Empedocles' hypothesis is 

 found in the De Generatione Animalium of Aristotle (Book 

 I, 722^, 20) : "in the time of his 'Reign of Love' says he 

 [Empedocles], 'many heads sprang up without necks,' and 

 later on these isolated parts combined into animals." 



Symbiosis, as illustrated by Davidson's speculation, means 

 a partnership between individual organisms of different 

 species so intimate as to make each member of the combina- 

 tion really dependent to some extent on the other. A con- 

 siderable number of such cases are now known in both bot- 

 any and zoology. Perhaps the most striking example is 



