CHAPTER XI 



GENETICS 



In the preceding three chapters have been examined the various 

 methods by which new individuals are brought into existence, the habits 

 which insure reproduction and the early care of the young individuals, 

 and the developmental processes which these young individuals undergo 

 in becoming adidt. The last of these topics (embryology) was discussed 

 in a general way, with reference only to the major features of animals, 

 those in which not only are all the members of one species alike, but in 

 which members of one large group of animals often closely resemble the 

 members of distantly related groups. However, development results 

 in the production of many details in which individuals may differ, but 

 which are just as definitely represented in the fertihzed egg as are the 

 nervous system, the digestive tract, or even the gastrula. The produc- 

 tion of these minor features and the laws that govern their occurrence are 

 the subject matter of the science of genetics. 



Scope of Genetics. — The word genetics means literally the origins of 

 things. Specifically, however, it is applied only to such origins as are 

 imphed in heredity, variation, determination of sex, and their apphca- 

 tions. Genetics is sometimes defined very broadly to include the origins 

 of the larger groups of animals from other groups; and in this sense it is 

 nearly synonymous with evolution. But such definitions have met with 

 opposition from those who study evolution by other than the experi- 

 mental method, with the result that the narrower use of the term is 

 most common. A more exact definition than the one implied in this 

 paragraph is not necessary for an elementary discussion, and would, in 

 fact, be difficult to formulate b fore the account of the facts and 

 mechanism of heredity in the following pages. 



The present day knowledge of heredity was gained through pains- 

 taking experiments upon hundreds of animals and plants, and from a 

 study of the cytology of the germ cells and the processes of embryonic 

 development. Out of the facts gleaned from these studies there has come 

 a knowledge of the mechanism by means of which a trait belonging to the 

 parents is so often reproduced in the offspring. A more perfect knowl- 

 edge of this mechanism is the principal immediate goal of the majority 

 of investigators of heredity, and it is being sought through the medium of 

 experiment and microscopic examination of the structures involved. For 



239 



