8o HEREDITY AND DESCENT. 



CHAPTER X. 

 HEREDITY AND DESCENT, 



IT is an everyday observation that an organism resembles 

 its parents. This tendency to structural resemblance in 

 genetically-related forms constitutes the principle of Heredity. 

 On the other hand, it is equally evident that the oifspring 

 is never identical in structure with either parent or even 

 its immediate ancestors ; there is always a structural diver- 

 gence which constitutes the principle of Variation. A 

 cursory inspection of a flock of sheep might fail to furnish 

 any individual variations, but a closer study would reveal 

 slight differences, which might enable one to discern the 

 particular sheep which had hereditary resemblances, or 

 were relatives of each other from others similarly con- 

 nected. A still more intimate acquaintance with them, 

 such as possessed by the shepherd, would lead to the 

 recognition of individual variations sufficiently definite to 

 identify each sheep at once by sight. In the ordinary 

 way the similarity due to heredity far outweighs the 

 dissimilarity due to variation. Hence a rabbit may 

 differ in small particulars, such as colour of hair or shape 

 of head, from its parents, but nevertheless it resembles them 

 in the vast preponderance of structural characters, which 

 we understand by the name *' rabbit." The individuals ex- 

 hibiting the small differences are often termed varieties^ those 

 exhibiting the more fundamental resemblances being termed 

 species. There is no real distinction between these two, as a 

 certain number of individuals of one species may form a 

 marked variety to which the appellation of a separate species 

 is a mere matter of opinion. 



The subject of heredity is intricate and it is difficult to lay down 

 any general law governing the principle. Individuals having the same 

 parents differ widely from each other. Some varieties or races have 

 a much greater power of transmitting their structure to their offspring 



