In the period following 1700 this difference gradually became apparent, and made necessary different treatment and distinct laws. This resulted from a recognition of the dissimilarity in character between property based on temporary service and that based on service for life. In the first place perpetual service gave rise to a new class of slaves. At first the only ones in Pennsylvania were such negroes as were imported and sold for life. But after a time children were born to them. These children were also slaves, because ownership of a negro held for life involved ownership of his offspring also, since, the negro being debarred by economic helplessness from rearing children, all of his substance belonging to his master, the master must assume the cost of rearing them, and might have the service of the children as recompense.[73] This was the source of the second and largest class of slaves. The child of a slave was not necessarily a slave if one of the parents was free. The line of servile descent lay through the mother.[74] Accordingly the child of a slave mother and a free father was a slave, of a free mother and a slave father a servant for a term of years only. The result of the application of this doctrine to the offspring of a negro and a white person was that mulattoes were divided into two classes. Some were servants for a term of years; the others formed a third class of slaves. In the second place perpetual service gave to slave property more of the character of a thing, than was the case when the time of service was limited. The service of both servants and slaves was a thing, which might be bought, sold, transferred as a chattel, inherited and bequeathed by will; but in the case of a slave, the service being perpetual, the idea of the service as a thing tended to merge into the idea of the slave himself as a thing. The law did not attempt to carry this principle very far. It never, as in Virginia, declared the slave real estate. In Pennsylvania he was emphatically both person and thing, with the conception of personality somewhat predominating.[75] Yet there was felt to be a decided difference between the slave and the servant, and this, together with the desire to regulate the slave as a negro distinguished from a white man, was the cause of the distinctive laws of the second period.