80 HEREDITY AND DESCENT. 
CHAPTER X. 
HEREDITY AND DESCENT. 
T is an everyday observation that an organism resembles 
itsparents. This tendency to structural resemblance in 
genetically-related forms constitutes the principle of Heredity. 
On the other hand, it is equally evident that the offspring 
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 varzeties, 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 
