alert stagnation

Yea Stacy, "Alert" doesn't seem to be an appropriate modifier for "stagnation," does it

More from Saki. Notice that the OP never gives context. This is tiresome.

This is no ordinary writer. Saki is well known for his ironic and playful style.

The Row is a place where people come to walk slowly and not do very much. It is "alive with" certain activities - Saki is being ironic here - it is full of people doing certain things, as the next section makes clear.

"The seekers after health, the seekers after notoriety and recognition, and the lovers of good exercise were all well represented on the galloping ground; the gravel-walk and chairs and long seats held a population whose varied instincts and motives would have baffled a social catalogue-maker. The children, handled or in perambulators, might be excused from instinct or motive; they were brought."

"Monotonous movement" should need no explanation; any dictionary should be able to explain the meaning of this phrase.

"Alert stagnation" - people were stagnating (not doing very much) but at the same time were watchful and alert.

These activities were appropriate to the time (a late June morning, when it would be hot) and place (a recreation and rest area)

That is a frequent lapse of the typical a2k OP, isn't it Con

For others such as I who don't know Saki

https://www.google.ca/#q=saki+monotonous+movement