282 



ANIMAL BIOLOGY 



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. An immense amount of experimental 

 work has demonstrated that, for practical purposes, the individual 

 may be regarded as an aggregate of essentially separate characters, 

 both structural and physiological, that are relatively stable and 

 may be inherited more or less as units. But the analysis does not 

 stop even at this level. Each character is regarded as represented in 



Fig. 179. — The evolution of the Game Cock. Results produced largely by 

 selection before our present knowledge of the mechanism of inheritance. 

 (From Metcalf, after Wright.) 



the chromosomes of the germ cells by one or more determining 

 factors, or genes; and whether or not a given character will be 

 present in a tree or a man depends upon whether the genes for this 

 particular character entered into the nuclear complex of the fer- 

 tilized egg which formed the individual. Therefore, geneticists are 

 now studying the relative positions which the genes occupy on 

 certain chromosomes; how they may cross-over from one chro- 

 mosome to the other of a synaptic pair ; how they mutually influence 

 one another, etc. (Fig. 196.) 



At present we are witnessing great advances in knowledge of 

 the underlying factors of heredity, and the data recently accumu- 

 lated are so vast that we can attempt here no more than to indicate 

 the character and promise of the principles already discovered. 



We may glimpse the field before us by a concrete example. 



