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, however, 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.

There is, however, a second class of variations which are not born in the individual, but which arise as the result of some conditions affecting its after-life.  The most extreme instances of this kind are mutilations.  Some men have only one leg because the other has been lost by accident.  Here is a variation acquired as the result of circumstances.  A blacksmith differs from other members of his race in having exceptionally large arm muscles; but here, again, the large muscles have been produced by use.  A European who has lived under a tropical sun has a darkened skin, but this skin has evidently been darkened by the action of the sun, and is quite a different thing from the dark skin of the dark races of men.  In such instances we have variations produced in individuals as the result of outside influences acting upon them.  They are not inborn, but are secondarily acquired by each individual.  We call them acquired variations.

It is not always possible to distinguish between these two types of variation.  Frequently a character will be found in regard to which it is impossible to determine whether it is congenital or acquired.  If a child is born under the tropical sun, how can we tell whether its dark skin was the result of direct action of the sun on its own skin, or was an inheritance from its dark-skinned parents?  We might suppose that this could be answered by taking a similar child, bringing it up away from the tropical sun, and seeing whether his skin remained dark.  This would not suffice, however; for if such a child did then develop a white skin, we could not tell but that this lighter-coloured skin had been produced by the direct bleaching effect of the northern climate upon a skin which otherwise would have been dark.  In other words, a conclusive answer can not here be given.  It is not our purpose, however, to attempt to distinguish between these two kinds of variations, but simply to recognize that they occur.