184 THE INDUCTIONS OF BIOLOGY. 



world there are types in which we see little more than 

 simple assemblages of Protophyta or of Protozoa — types in 

 which the units, though coherent, are not differentiated but 

 constitute a uniform mass. In treating of structure we are 

 not here concerned with these unstructured types, but may 

 pass on to those aggregates of protoplasts which show us 

 differentiated parts — Metaphyta and Metazoa: economizing 

 space by limiting our attention chiefly to the last. 



When, half a century ago, some currency was given to the 

 statement that all kinds of organisms, plant and animal, 

 which our unaided eyes disclose, are severally composed of 

 myriads of living units, some of them partially, if not com- 

 pletely, independent, and that thus a man is a vast nation of 

 minute individuals of which some are relatively passive and 

 others relatively active, the statement met, here with in- 

 credulity and there with a shudder. But what was then 

 thought a preposterous assertion has now come to be an 

 accepted truth. 



Along with gradual establishment of this truth has gone 

 gradual modification in the form under which it was origi- 

 nally asserted. If some inhabitant of another sphere were 

 to describe one of our towns as composed exclusively of 

 houses, saying nothing of the contained beings who had built 

 them and lived in them, we should say that he had made a pro- 

 found error in recognizing only the inanimate elements of 

 the town and disregarding the animate elements. Early 

 histologists made an analogous error. Plants and animals 

 were found to consist of minute members, each of which ap- 

 peared to be simply a wall inclosing a cavity — a cell. But 

 further investigation proved that the content of the cell, 

 presently distinguished as protoplasm, is its essential living 

 part, and that the cell- wall, when present, is produced by it. 

 Thus the unit of composition is a protoplast, usually enclosed, 

 with its contained nucleus and centrosome. 



§ 54c. As above implied, the individualities of the units 



