Chapter IX 

 HEREDITY 



Species. Biologists look upon all living objects as forming a 

 great unity made up of a wide variety of more or less remotely 

 related groups, each group being divisible into smaller groups 

 within which the resemblances are closer; increasing preponderance 

 of similarities mark still more closely related subdivisions, until the 

 ultimate indivisible unit is an individual animal or plant. The forms 

 that most closely resemble an individual animal or plant are its 

 parents or immediate offspring. A group within which the resem- 

 blances are so close that the various members may stand in the 

 relationship of parent and offspring, or brother and sister, is com- 

 monly called a species. This is a general definition and is generally 

 applicable, but within some groups commonly recognized as species 

 some varieties are so distinct that they may be, and often are, legit- 

 imately regarded as separate species. Moreover, interbreeding some- 

 times occurs between recognized species. 



Offspring always resemble parents but also always differ in some 

 respects. To discuss the question as to the nature and mechanism 

 of inheritance of parental qualities and as to the origin and nature 

 of the differences between offspring and parent involves us in a 

 whole series of other questions that constitute the complicated 

 subject of heredity. Here we shall concern ourselves with inheritance 

 within the species; hence it has been necessary to repeat and enlarge 

 the definition of the word. The origin of species will be discussed 

 elsewhere. 



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