CHAPTER 17 

 GENETICS 



The object of the study of embryology, as outHned in the preceding 

 chapter, is to discover how animals become what they are. Mere obser- 

 vation shows by what steps the development proceeds, and experiments 

 have revealed some of the physiological principles underlying these 

 events. Embryology discloses these things satisfactorily for the major 

 features of structure which are essentially alike in whole large groups 

 of animals — satisfactorily, that is, if one does not require to know the 

 fundamental causes of the different types and steps of development. 



There are, however, many minor features of organization which are 

 just as definitely fixed parts of animals as their digestive and nervous 

 systems are, but which are different in different individuals. Color of 

 eye, shape of hair, dimples, stature, complexion, and talents are different 

 in different people, yet all through their embryonic development it is 

 quite settled what these characters are going to be. No embryologist 

 could tell what the outcome would be in any of these traits in the adult, 

 but the die would have been cast before cleavage of the egg had begun. 



These individual differences furnish another way of learning the rules 

 governing the development of characteristics. This method consists 

 of crossing individuals having different traits and observing the occur- 

 rence of these different traits among the descendants. This is the method 

 of genetics. It would be impossible to use it to discover much about 

 the structures with which embryology deals, for there is no difference 

 between individuals ^vith respect to the major features. Nevertheless 

 when the mode of inheritance of minor characters has been discovered, 

 it may be taken as certain that the inheritance of the major features 

 follows the same scheme. Genetics uses minor features to discover the 

 principles of heredity, with the conviction that the same principles apply 

 to the major features as well. 



Genetics has the further advantage, in the study of origins, that it 

 reveals more fundamental causes. While embryology, when it employs 

 experiment, may reveal physiological processes (;ausing the developmental 

 changes, genetics lays bare to some degree the causes of the physiological 

 processes. It is today one of the biologist's most })otent tools in delving 

 into the fundamental nature of living things. Embryology is an aid 

 because it reveals some of the visible mechanism of heredity, particularly 



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