110 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



immediate mitochondrial origin of chloroplasts is opposed 

 to the acceptance of a genetic relationship to the blue- 

 green algae. On further reflection and analysis, however, 

 such a presumption is hardly vaUd. The chloroplasts 

 have in the main the characteristics of Uving bodies. Cell 

 histogenesis in ontogenetic development demonstrates the 

 possibihties of differentiation. It is well known that the 

 germ cell contains potentialities for the elaboration of all 

 the tissues of an individual, whether it be a man or a worm. 

 These "potentialities" are not sufficient within themselves 

 to produce the completed individual. It recently has been 

 emphasized by Child ('24) and others, that environmental 

 factors play an important role in histogenetic differentia- 

 tion of tissues and organ formation. These environmental 

 factors, perhaps, are chiefly of a chemical nature, although 

 mechanical and physical factors are known to be involved. 

 The metaboHc products of one organ or tissue may be the 

 stimulating influence that inaugurates differentiation in 

 other organs or tissues. According to Stockard ('21), 

 the active differentiation of a tissue or the formation of an 

 organ may exercise an inhibitory influence on the differen- 

 tiation of a number of other organs and tissues. 



Conkhn ('19) has given a clear exposition of the status 

 of histogenetic development in the following words: "to a 

 certain extent differentiations are already present in the 

 germ cells, but during the course of development their 

 number is greatly increased. This increase in differentia- 

 tion is caused by modification of previously existing 

 differentiations, by transformation rather than by formation 

 de novo, by evolution rather than by creation. New 

 things appear in the course of ontogeny because of 

 new combinations of things already present, just as new 

 chemical substances with new qualities appear as a result 

 of new combinations of elements." 



The genetic origin of chloroplasts and other plasts from 



