CONCLUSIONS AND SUGGESTIONS 193 



wise the result of physiological isolation. Elsewhere I 

 have endeavored to show that physiological isolation is 

 a fundamental factor in asexual reproduction in both 

 plants and animals, and that reproduction results from 

 physiological isolation because the isolated part loses 

 to a greater or less extent its differentiation as a part, 

 becomes physiologically younger, and undergoes a new 

 individuation.^ In chap, iv above it was also pointed 

 out that agamic reproduction in Tubularia and Planaria 

 is readily interpreted as the result of physiological 

 isolation. Moreover, in the discussion of the data of 

 experimental reproduction we have seen that physio- 

 logical isolation and reproduction may result, not only 

 from increase in size beyond the range of dominance, 

 but also from decrease in the range of dominance in 

 consequence of decrease in metabolic rate in the domi- 

 nant region, from decrease in conductivity in the path of 

 transmission, and finally from a decrease in receptivity 

 of a subordinate part, brought about by the action of 

 local factors, which determine the establishment of new 

 gradients in it or make it otherwise more independent. 

 Undoubtedly all these different forms of physiological 

 isolation occur in nature, and in many reproductive 

 processes more than one of them are probably concerned. 

 Reproduction in consequence of increase in size is 

 one of the commonest forms of reproduction in organic 

 individuals from the single cell to complex organisms 

 among both animals and plants. Reproduction also 

 occurs very commonly under conditions unfavorable to 



^ Child, " Die physiologische Isolation von Teilen des Organismus," 

 Vortrdge und Aufsatze uber Entwickehmgsmechanik , H, XI, 191 1; Senes- 

 cence and Rejuvenescence, 1915, pp. 228. 



