Passing now to the causes of variation, all of the countless deviations of living things can be referred to three kinds of primary factors; namely, the environmental, functional, and congenital influences that work upon the organism in different ways and at different times during its life.  We shall learn that the evolutionary values of these three classes are by no means equal, but we take a long step forward when we realize that among the things we see every day are facts demonstrating the reality of three kinds of natural powers quite able to change the characters of organic mechanisms.

The “environment” of an organism is everything outside the creature itself.  In the case of an animal it therefore includes other members of its own kind, and other organisms which prey upon its species or which serve it as food, as well as the whole series of inorganic influences which first come to mind when the term is used.  For example, the environment of a lion includes other lions which are either members of its own family, or else, if they live in the same region, they are its more or less active rivals and competitors.  In the next place, other kinds of animals exist whose lives are intimately related to the lion’s life, such as the antelopes or zebras that are preyed upon, and the human hunter to whom the lion itself may fall a victim.  In addition, there are the contrasted influences of inorganic nature which demand certain adjustments of the lion’s activities.  Light and darkness, heat and cold, and other factors have their direct and larger or smaller effects upon the life of a lion, although these effects are less obvious in this instance than in the case of lower organisms.

The reality of variations due to the inorganic elements of the environment is everywhere evident.  Those who have spent much time in the sun are aware that sunburn may result as a product of a factor of this class.  The amount of sunlight falling upon a forest will filter through the tree-tops so as to cause some of the plants beneath to grow better than others, thus bringing about variations among individuals that may have sprung from the myriad seeds of a single parent plant.  In times of prolonged drought, plants cannot grow at the rate which is usual and normal for their species, and so many variations in the way of inhibited development may arise.

Then there are the variations of a second class, more complex in nature than the direct effects of environment,—­namely, the functional results of use and disuse.  A blacksmith uses his arm muscles more constantly than do most other men, and his prolonged exercise leads to an increase of his muscular capacity.  All of the several organic systems are capable of considerable development by judicious exercise, as every one knows.  If the functional modifications through use were unreal, then the routine of the gymnasium and the schoolroom would leave the body and the mind as they were before.