CHAPTER XVIII 

 THE CONTINUITY OF LIFE 



Owing to the imperfection of language the offspring is termed a new 

 animal, but is in truth a branch or elongation of the parent. 



— Erasmus Darwin. 



Since so far as is known all life now arises from preexisting life 

 and has done so since matter first assumed the living state, it 

 apparently follows that the stream of life is continuous from the 

 remote geological past to the present and that all organisms of 

 to-day have an ancient pedigree. It is to the establishment of this 

 as the reasonable conclusion from the data accumulated during 

 recent years, that from now on our attention is somewhat more 

 particularly directed ; and accordingly it is necessary first of all to 

 consider in some detail the relation of parent to offspring in present- 

 day forms as exhibited by reproduction. 



A. Reproduction 



The power of producing new individuals specifically similar to 

 the parent is, as has been seen, one of the most important character- 

 istics of living in contrast with lifeless matter. Furthermore, re- 

 production is typically cell division. This is quite evident in uni- 

 cellular plants and animals, but by no means so obvious in higher 

 organisms where, as we know, special gonads and highly complex 

 accessory organs are developed in furtherance of reproduction. 



It will be recalled that in Paramecium, for example, the nucleus 

 and cytoplasm divide into two parts, so that by cell division, here 

 called binary fission, the identity of the parent organism is 

 merged into the two new cells. Simple as this seems, the fission 

 of Paramecium actually involves considerably more than the halv- 

 ing of the original cell, because, as a matter of fact, each half 

 must reorganize into a complete new individual with all parts 

 characteristic of the parent. (Figs. 8, 28.) 



Among some unicellular animals (e.g., the Sporozoa) the parent 

 cell, instead of merely forming two cells by binary fission, becomes 

 resolved into many cells by a series of practically simultaneous 



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