280 GENERAL ZOOLOGY 



represented largely by the cytoplasm. If the cytoplasm pro- 

 vides a necessary condition for the realization of the hereditary 

 potencies of the genes, it becomes highly important in heredity. 

 There is always a cytoplasmic inheritance from the egg, but its 

 analysis into specific factors is difficult because its organization is 

 not expressed in easily recognized morphological forms. The 

 organization of the egg cytoplasm is a property of the hyaline 

 ground substance of the egg, rather than the visible formed 

 structures such as yolk, fat, mitochondria, etc. Unfortunately 

 for purposes of microscopic analysis, the ground substance of the 

 cytoplasm is optically homogeneous and contains no units com- 

 parable to chromosomes. Nevertheless, it is organized and an 

 understanding of this organization is essential to an under- 

 standing of the action of genes in development. An animal 

 inherits a certain protoplasmic organization permeating cyto- 

 plasm as well as nucleus; the final outcome of development 

 being a result of the reaction of the organized plasm with the 

 environment. 



Some biologists regard the gene as the ultimate unit of life. 

 It is thought to be protein in nature, capable of reproducing itself 

 and carrying on the usual metabolic activities ordinarily asso- 

 ciated with life. The cell is a medium in which its properties 

 can be manifested. Within recent years it has been possible to 

 obtain in a pure crystalline form the virus causing mosaic disease 

 in tobacco plants. This virus is also protein in nature and is 

 capable of reproducing itself. It has been suggested that the 

 virus and the gene are possibly of the same general nature and 

 that they represent a simple form of living system. If so, the 

 first form of life evolved from nonliving nature may have been 

 a system of this sort. The thought is suggestive in that it aids 

 in partially bridging the gap between nonliving chemicophysical 

 systems and typical cellular living systems. 



Human Heredity. — Inheritance in man obeys the same laws 

 of heredity that hold for other forms of life. Eye color, hair 

 color, morphological peculiarities and physiological traits of 

 various sorts are inherited. From the standpoint of their value 

 to human society the inheritance of some human traits are far 

 more important than others. The science of the improvement 

 of the human race by better breeding, or as Francis Galton, the 

 author of the term, put it, "the science which deals with all 



