Analogies to such conditions are given in plants.

In certain plants it has been shown that from unknown causes there appears a tendency to the production of variations.

A very beautiful herbaceous peony known as "Bridesmaid" after having grown for a number of years in single form, in one year wherever grown suddenly became double.

The peculiar thing with the lower unicellular organisms is that the changes which so arise do not tend to become permanent, the organism reverts to its usual character, the disease to its sporadic type.
A very fatal form of poliomyelitis has for a number of years prevailed in Sweden.

In the United States there have been continually a number of single cases of the disease, and it is not impossible that a more pathogenic strain of the organism has developed in Sweden and has been imported into this country, giving rise to the much greater extension of the disease in a number of places.
The most cursory study of the infectious diseases shows that there is great variation in the susceptibility of individuals.