THE BEGINNINGS OF LIFE. 



143 



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 :als of thest 

 id with thm 

 'r organizit 

 limals, agait, 

 ;e from 



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reproduce 



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 om has bee^ 



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of late 



within the tissues of plants and animals — there is at 

 present, and has been taking place, a corresponding 

 evolution of living forms^ or morphological units. This 

 enquiry will involve a consideration of the present 

 aspect of the ^Cellular theory V of organization, and a 

 sketch of the principal modifications which, 

 years, that doctrine has undergone. 



Facts are still multiplying day by day which tend, 



« 



to show that the elements of the tissues in man and 

 in the higher animals are possessed of an inherent 

 power and activity of their own — of a separate indi- 

 viduality in fact, though one which is subordinate to 

 the higher and more complex individuality of the 

 organism to which they belong, and as parts of which 

 they have been evolved. Tissue elements, such as 

 epithelial cells, are to a certain extent like distinct 

 organisms. They have a definite Life of their own 

 longer or shorter according to the situation in 

 which they occur, and which is therefore very vari- 

 ously related to that of the whole organism. Their 

 individuality of character or function is, moreover, 

 further shown by the power which they possess of 

 selecting their own peculiar nutritive elements out 

 of a complex fluid, or nutritive blastema— the blood 

 common to all parts of the organism. But, granting 

 all this, the question then comes for consideration as 

 to whether we are to look upon <^ Cells' as the invariable 

 and ultimate morphological units— whether they alone 

 can exhibit those subordinate vital activities upon 



