ox THE NATURE OE CELL-ORGAN/Z AT/ON. 97 



that opportunity, by one means or another, of their combining 

 thus and death is the result, somewhat in the same way as in 

 the case of an obhgatory parasite, which though descended 

 from a free-living ancestor, dies if deprived of its appropriate 

 host. 



The minute organisms which we assume to make up the 

 cytoplasm on the one hand and nucleus on the other, probably 

 once were free and lived independently, but if so, it is plain 

 they have since lost their power by the acquisition of sym- 

 biotic habits. By adopting symbiotic habits, however, they 

 acquired the ability to adapt themselves to surroundings so 

 different from the normal habitat of each that neither symbiont 

 alone could have lived in them, and thus possibilities each was 

 incapable of accomplishing alone, are performed successfully 

 by the composite organism. ^ We can reasonably suppose, 

 therefore, that those cell- forming organisms, which entered into 

 the symbiotic relation in the past, zvith others, have survived in 

 a modified form, in the body of the nucleated cell, zvhile those 

 oro-anisms zvhicJi did not, have perished ozving to their iiiability 

 to adapt themselves to the vicissitudes of circumstances. The 

 nucleated cell, then, is a colony of heterogeneous organisms, 

 which maintains a complete autonomy and behaves as if it were 

 an independent organic being, subject to the law of growth and 

 development peculiar to itself. 



It is perhaps needless to point out, after we have dwelt so 

 much on the subject, that the fundamental assumption of our 

 theory is that at least some of the earliest living beings that 

 ■ever existed ivere not in the form of a cell, but a great deal 

 simpler, somezvhat like those individual physiological units 

 zuhich constitute the cytoplasm and the nucleus of the cell. The 

 cell itself was formed later out of these still smaller organisms 

 which already existed. It is not our purpose, in this place, to 

 enter into an extensive examination of the different views that 

 have been brought forward to explain what these cell-forming 

 units are. All that is essential for our present purpose is the 

 existence, as all writers unanimously agree, of such units in 



1 See important remarks on the result of commensalism between two organisms. 

 Sachs : Physiology of Plants, pp. 391-394. 



