158 THE STORY OF THE LIVING MACHINE. 



generations differ from each other in one respect 

 or another. Birds vary in the length of their 

 bills or toes; butterflies, in their colours; dogs, 

 in their size and shape and markings; and so 

 on through an endless category. Plants and 

 animals alike throughout nature show variations 

 in the greatest profusion. It is these variations 

 which must furnish us with the foundation of the 

 changes which have gradually built up the living 

 machine. 



Of the fact of these variations there is no 

 question, and the matter need not detain us. 

 Every one has had too many experiences to ask 

 for proof. Of the nature of the variations, how- 

 ever, there are some points to be considered which 

 are very germane to our subject. In the first place, 

 we must notice that these variations are of two 

 kinds. There is one class which is born with the 

 individual, so that they are present from the time 

 of birth. In saying that these variations are born 

 with the individual we do not necessarily mean 

 that they are externally apparent at birth. A 

 child may inherit from its parents characters which 

 do not appear till adult life. For example, a child 

 may inherit the colour of its father's hair, but this 

 colour is not apparent at birth. It appears only 

 in later life, but it is none the less an inborn 

 character. In the same way, we may have many 

 inborn variations among individuals which do 

 not make themselves seen until adult life, but 

 which are none the less innate. The offspring of 

 the same parents may show decided differences, 

 although they are put under similar conditions, 

 and such differences are of course inherent in 

 the nature of the individual. Such variations are 

 called congenital variations. 



