for all progress and all degeneration. No advance would be possible, if offspring had always exactly resembled their ancestors.
Variations are usually divided into two main kinds. Between these there are important differences.
(1) Some variations are inborn. Environment has had no influence in their production. They are inherent in the constitution of the individual, and belong to it at birth. These variations appear abruptly in the child. They cannot be accounted for by the habits or surroundings of the parents of the child in whom they make their appearance. Biologists call these variations discontinuous variations or "sports." They have collected much evidence to show that these discontinuous variations occur on a large scale among plants and animals. In human beings also there are instances. The child who is born with great musical abilities may be the son of parents quite devoid of them. Nearly every genius is a "sport" or discontinuous variation. When once discontinuous variations have occurred, they may be transmitted to offspring, and thus become a permanent heritage of the species.
(2) Other variations are acquired. They are not born with the child: the child acquires them during its life by interaction with its environment. They are developed by the child under some external stimulus, such as climate or injury. Thus they show the direct influence of the environment. If the child be brought up in a tropical country, he may acquire a sallowness that will last for life. Through injury he may lose the use of a limb.
