28 THE PROBLEMS OF EVOLUTION 



The simplest biological unit which is known to 

 carry on independent life is the cell. Regardless 

 of the fact that bacteria present certain character- 

 istics which distinguish them from the vast ma- 

 jority of cells, this statement still applies to living 

 matter generally. Until proof to the contrary is 

 available we may regard bacteria as true cells of 

 a special, perhaps a more primitive, type. The 

 cells with which we are concerned elsewhere in 

 the organic world, however, are definitely organ- 

 ized and complex in themselves. They are made 

 up of various subsidiary units. The nucleus is one, 

 but it in turn includes lesser units such as the 

 chromosomes, and beyond these limits we are 

 justified in believing that the genes may also be 

 definite entities. The cytoplasm presents a variable 

 host of minor units, granules, mitochondria, and 

 others. Nevertheless, since these minor particles 

 are incapable of existing independently, we may 

 accept the cell as the ultimate living unit. 



Were cells always to exist as independent organ- 

 isms, as in the Protophyta and Protozoa, we would 

 lack one valuable bit of evidence, but when they 

 band together intimately and inseparably in the 

 multicellular organism the case is different. Here 

 we have a concrete entity, a unit which can be 

 seen, heard, felt, smelled, and tasted. There is no 

 obstacle to its acceptance as a natural entity, re- 

 gardless of one's interpretation of the word, al- 



