14 INTRODUCTION 
visible differences between them may seem very small 
to an untrained eye. 
Lastly, evidence of a confirmatory nature may be 
obtained by observing the results of cross-fertilization 
between a pair of closely allied species. Much, too, 
may be made out from the failure of such experiments, 
since the refusal of two plants to breed together is 
generally regarded as clear evidence of their specific 
distinctness. But for this reason the method of cross- 
breeding is more particularly adapted for the examina- 
tion of forms somewhat nearly related to one another 
—for example, different members of the same species. 
As the result of the methods presently to be 
described, the fact has been established that two 
entirely distinct sorts of divergencies may appear 
among members of a single family. Variations, that 
is to say, may be of at least two different kinds. In 
the first place we have those slight differences which 
invariably distinguish all the members of every family 
—individual variations which affect every part and 
every character. Such differences are known as 
fluctuating, normal, or continuous variations. As an 
example we may cite the variations in size or stature 
shown by the various members of any purely-bred race. 
When a large number of individuals are compared in 
respect of a character of this kind, they are found to 
fall into a continuous series ranging from a certain 
extreme of shortness on the one hand to an extreme 
of tallness on the other. Individuals of a medium 
height, however, are usually more numerous than either 
