456 SCIENTIFIC THOUGHT. 



Growth by intussusception and assimilation has long 

 been recognised as the characteristic property of all 

 living matter, of every living cell Mechanical causes 

 suffice to explain the further process of division as a 

 necessary consequence of continued growth, the forma- 

 tion of new cells out of existing ones, the process of 

 reproduction. Only in the lower organisms, however, 

 does reproduction exist simply as multiplication by 

 division. In all higher organisms at least, reproduction 

 by division seems connected with the phenomenon 

 of death of a portion of the dividing organisms : a 

 differentiation seems to set in between the new cells, 

 some gradually losing their power of self-multiplication 

 by division, and thus being doomed sooner or later 

 to arrive at the end of their organic existence ; while 

 others retain this power or regain it by uniting with 

 others the process of fusion of male and female elements 

 and seem thus to be specially endowed with the work 

 of reproduction i.e., the preservation of the continuity of 

 life. The great morphologist Eichard Owen, about the 

 middle of the century, in a tract on Parthenogenesis, 

 remarked that "not all the progeny of the primary 

 impregnated germ-cell are required for the formation of 

 the body in all animals : certain of the derivative germ- 

 cells may remain unchanged and become included in 



embryological development, assisted 

 or disturbed by experiments carried 

 on in microscopic dimensions. I 

 recommend, besides the larger works 

 of Hertwig and Roux already re- 

 ferred to, the highly suggestive 

 writings of Hans Driesch, notably 

 his 'Analytische Theorie der or- 

 ganischen Entwickelung ' (1894). 



and 'Die Biologic als selbstandige 

 Grundwissenscbaft' (1893). As a 

 very helpful introduction to the 

 original views of this writer, Eng- 

 lish readers will welcome the con- 

 cluding chapter of Prof. E. B. 

 Wilson's book. The Cell in Develop- 

 ment and Inheritance' (1896). 



