CHAPTER VII. 



APPLICATION OF THE THEORY TO MULTICELLULAR 

 ORGANISMS, AND THE RELATION OF THE GERM- 

 CELLS TO THE BODY. 



If we admit the efficacy of the causes of which 

 we have been treating, and beHeve that they have 

 acted in the described manner so as to produce the 

 diversity noticed among the single-celled protozoa, 

 and also to account for the simple phenomena of 

 heredity among them, then we are immediately led 

 on to inquire how a theory, which may be plausible 

 in regard to organisms composed of single cells, 

 will appear when applied to those organisms which 

 are each composed of many cells. Evidently, if there 

 be a physiological difference between the many-celled 

 and the single-celled organisms, their organic phenom- 

 ena cannot be explained by the same theory. There 

 has been a general impression among biologists, 

 that the dividing line between the metazoa and the 

 protozoa is peculiarly unique among all the divisions 

 in the classification of organisms, and that it indi- 



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