20 EVOLUTION OF CELLULAR STRUCTURES. 



species to external environmental causes as to arbitrary mechanisms 



of heredity. 



The extent to which the static concept of a normally unchanging 

 heredity has obscured evolutionary thought and investigation could not 

 be better shown, perhaps, than by the fact that, notwithstanding the 

 great multiplicity of terms which have been proposed for all the 

 imagined kinds of variations, no name has been suggested for this nor- 

 mal and necessary intraspecitic diversity. The deficiency may be made 

 good by the use of the word heterism for the whole group of phenomena, 

 ranging from mere individual diversity to the highest specializations 

 of heterism, exemplitied by sexes, castes, and polymorphic conditions. 

 It is true that the members of a species look alike Avhen compared with 

 those of other species, and there may be no harm in ascribing this 

 likeness to heredity, but there is nothing to show that this heredity of 

 general resemblance has anything to do with evolution except as an 

 incidental result. Evolution does not take place between species, but 

 inside of them; it is not an //^fenspecific phenomenon, but /^^i^mspecific. 

 Its principal factors are heterism and symbasis, not heredity and envi- 

 ronment, as believed by the selectionists, nor heredity and segrega- 

 tion, as supposed by the nmtationists. 



HEREDITY IN RETICULAR DESCENT. 



The greater efficiency of the double nuclei is, however, only one 

 more evidence of the importance of sex as a means of diversity and of 

 bringing diverse protoplasms together. The nuclear network of 

 chromatin which controls the activities of the cell corresponds to the 

 network of descent through which the cell has come into existence. 

 Symbasis, or diversity of descent with normal interbreeding, is the 

 foundation of the strength and \'itality of the organism, because it 

 increases the efficiency of the luiclei of the component cells. 



Inbreeding or defective fertilization, on the other hand, would cause 

 nuclear deterioration, as so strikingly shown by Maupas in the so-called 

 senile degeneration of ciliate Infusoria induced by keeping them too 

 long without cross-fertilization. This phenomenon is, indeed, closely 

 parallel to senile degeneration, but there is, nevertheless, an important 

 difference. In true senile degeneration the vigor of the cells is declin- 

 ing because of the absence or long postponement of conjugation. In 

 monobasic degeneration, conjugation may take place, but is not effective 

 because of insufficient diversity of descent. Monobasis is the antithesis 

 of symbasis; it means descent without cross-fertilization, on single or 

 very narrow lines. The inevitable result is degeneration, with a 

 rapidity proportional to the closeness of the inbreeding and the com- 

 plexity of the organisms. 



This intimate relation between organic descent and organic structure 

 enables us to understand the phenomena of organic succession without 



