According to Wikipedia:

“The Selective Service Act or Selective Draft Act Pub.L. 65–12, 40 authorized the federal government to raise a national army for the American entry into World War I through conscription. It was envisioned in December 1916 and brought to President Woodrow Wilson’s attention shortly after the break in relations with Germany in February 1917.”

Moses Alvin Clark registered on 12 Sep 1918 in Wilton, Franklin County, Maine.  He was 37 years old, working as a blacksmith for Daniel B. Rowe, also of Wilton. He was registered by Fred E. Trefethin, who was, I believe, a physician in Wilton at that time.

He was described as being of “medium height, medium build, with brown eyes and black hair. No physical disqualification.”

The WWI Registration cards provide us with a way to actually visualize our ancestors as young people. For example, when I knew my great-grandfather, he had almost no hair at all. The old sepia-toned photographs I saw as a young girl showed that he had dark-hair, but I would not have guessed that his hair was black.

Medium height, medium build we knew. However, everyone in the family also knew to take care when clasping hands with Moses Clark, even in his 90s. He had an impressive grip, born of swinging a heavy iron hammer as a blacksmith. My mother used to take her rings off before holding his hand.

SOURCE: Ancestry.com. U.S., World War I Draft Registration Cards, 1917-1918 [database on-line].