262 FOUNDATIONS OF BIOLOGY 



species and the origin of individuals are essentially the same 

 question. If we can solve the relations of parent and off- 

 spring, the origin of species will largely take care of itself. As 

 a matter of fact, historically the question of species origin was 

 approached first, and through the work of Darwin became 

 of paramount interest in the latter half of the nineteenth 

 century. The twentieth century finds the individual the 

 genetic relation of parent and offspring the center of 

 investigation, and it forms the science of genetics. OR- 

 GANIC EVOLUTION established the general fact that all or- 

 ganisms are related by descent; GENETICS attempts to show 

 how specific individuals are related. 



Even further has the pendulum swung from the general 

 to the particular. To-day the most intense investigation is 

 centered not on the heritage of the individual as a whole, 

 but on particular characters of the individual. The concept 

 has arisen from recent experimental work that, for practical 

 purposes, the individual may be regarded as congeries of 

 UNIT CHARACTERS, both structural and physiological, which 

 are more or less stable, and which are inherited as units. 

 But the analysis does not stop even at this level. There 

 seems to be good reason to believe that each so-called unit 

 character is represented in the chromosomes of the germ ceils 

 by a definite factor, determiner, or, as it is now usually 

 termed, GENE; and whether or not a given character will be 

 present in a tree or a man depends upon whether the gene for 

 this particular character entered into the nuclear complex of 

 the fertilized egg which formed the individual. Therefore, 

 geneticists are busy plotting the relative positions which 

 these genes occupy on certain chromosomes and how they 

 may ' cross-over' from one chromosome to the other of a 

 synaptic pair. 



Although at present we are apparently at the threshold of 



