The question of the inheritance of characteristics which the parents have acquired as the result of the action of environment upon them is one which is still actively investigated by the students of heredity, but the weight of evidence is opposed to this belief.
In the new organism the type of the species is preserved and the variations from the mean to which individuality is due are slight.

We are accustomed to regard as variations somewhat greater departures from the species type than is represented in individuality, but there is no sharp dividing line between them.
Very much wider departures from the species type are known as mutations.

Such variations and mutations, like individuality, may be expressed in qualities which can be weighed and measured, or in function, and all these can be inherited; certain of them known as dominant characteristics more readily than others, which are known as recessive.

If these variations from the type are advantageous, they may be preserved and become the property of the species, and it is in this way that the characteristics of the different races have arisen.
Certain of the variations are unfavorable to the race.

The varying predisposition to infection which undoubtedly exists and may be inherited represents such a variation.