Why Did The Danellians Establish The Time Patrol?

The Danellians are the evolutionary successors of homo sapiens in a variable timeline. Manse Everard's class at the Patrol Academy is told that:

"'They were simply concerned with protecting their own existence... the Patrol was set up to police the time lanes.'"

-Poul Anderson, "Time Patrol" IN Anderson, Time Patrol (Riverdale, NY, December 2010), pp. 1-53 AT 2, p. 11.

However, Wanda Tamberly wonders whether it was altruism that had motivated the Danellians to establish a temporal police force that both helps time travellers and protects the history that leads to them, the Danellians:

"For them this may be a matter of simple survival. They have never told us, we hardly ever see them, we do not know."

-Poul Anderson, The Shield Of Time (New York, July 1991), PART FOUR, 1965 A. D., pp. 163-164.

Emphases change as the series proceeds. Does the series commence in one timeline and end in another? Does it read like an incomplete trilogy? A final volume to tie everything together would have been welcome.