PROTOPLASM 41 



similarly birds and mammals must have something that is lack- 

 ing in amphibia and reptiles as well as in fish. In modern genetics 

 it is maintained that a specific characteristic of an adult is repre- 

 sented in the fundamental organization by an hypothetical entity 

 which geneticists term a gene.* The evidence for this is con- 

 vincing, as is also the conclusion that the seat of genes is in the 

 nucleus, in which genes are found to be located in the chromo- 

 somes — those characteristic bodies which appear at the division 

 period of cells. 



On such an hypothesis the chromosomes of a mammal must 

 be exactly as much more complex than the chromosomes of an 

 amphibian as the adult mammal is more complex than the adult 

 amphibian. In other words, the evolution of the fundamental 

 organization must have kept pace with the evolution of the 

 derived organization, not necessarily by an increasing complexity 

 of the genes, but presumably by the increasing complexity of 

 gene associations. 



These general considerations in biology are applicable not 

 only to the higher types of animals and plants, but to the small- 

 est living things as well — the protozoa, or unicellular animals, 

 and the protophyta, or unicellular plants. Here also the distinct 

 types of organization permit a classification upon natural (phylo- 

 genetic) lines, although the relationships of the various forms 

 are often obscure and frequently conjectural. The idea has been 

 advanced from the evidence revealed by detailed microscopic 

 studies of the cells, that in some protozoa {e.g., Uroleptus hal- 

 seyi) there is only one type of gene per chromosome. The devel- 

 opment and differentiation, in such a simple organism, of the 

 fundamental organization into the adult derived organization, 

 although accomplished within the confines of a single cell, are 

 nevertheless unmistakable. 



* See "Heredity and Variation," pages 52-54 and 65, of this Series. 



