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 two quite 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 



