Your lamb shall be without blemish. Natural piety would teach that "the blind, the lame, and the sick" should not be selected for sacrifice (Malachi 1:8). The Law afterwards expressly forbade any blemished animals—"blind, or broken, or maimed, or having a wen, or scurvy, or scabbed"—to be offered for any of the stated sacrifices, though they might be given as free-will offerings (Le Exodus 22:20-25).
The absence of blemish was especially important in a victim which was to typify One "holy, harmless, undefiled, separate from sinners." A male. As standing in place of and redeeming the first-born of the males in each family.
Of the first year. Perhaps as then more approaching to the ideal of perfect innocence. The requirement was not a usual one. Or from the goats. Theodoret says the proviso was made for the relief of the poorer class of persons; but practically it seems not to have taken effect.
When people were poor, their richer neighbours supplied them with lambs (Kalisch).