CHAPTER XII. THE RABBIT'S PLACE IN THE 

 ANIMAL KINGDOM. 



1. Heredity. Innumerable wild rabbits exist in 

 England, and if we study any one of these we find the 

 same general structure, the same functions in the various 

 organs, the same instincts and mode of life. Such a series 

 of closely similar animals forms what is called an animal 

 species ; we will discuss the exact meaning of this term later 

 on. This general resemblance of all rabbits to one another 

 is usually explained by saying that they are all the offspring 

 of parents which were themselves rabbits, and these in turn 

 of other ancestral' rabbits, until if we trace the history of 

 rabbits far enough back we might conceivably find that all 

 were descended from a single pair of rabbits. This explan- 

 ation does not really explain anything : it merely brings the 

 fact to be explained within the generalization that all new 

 animals tend to resemble their parents. This tendency is 

 called heredity. We have seen that the ultimate causes of 

 heredity probably are contained in the nuclei of the repro- 

 ductive cells which unite to form the fertilized ovum from 

 which every rabbit is developed. What the nature of those 

 causes may be we can hardly guess at present. 



2. Variation. We have spoken of heredity as a 

 tendency, and no more definite term can be applied to it, for 

 not only does it resolve itself into two partly-opposing 

 tendencies towards resemblance of one or the other of the 

 two parents that are necessary to produce a fertilized ovum, 

 but there always seem to be present other tendencies 

 towards difference from both parents. Indeed, it is probable 

 that out of the enormous numbers of rabbits that live or 

 have lived, no two have ever been exactly alike. Even in 

 our description of the rabbit, which has been limited to the 

 broadest facts of structure and function, we have had to 

 mention that some rabbits have twelve pairs of ribs, others 



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