It is merely a caution to look away from use and disuse to another explanation of variation that is not yet forthcoming.
After all, the plain man must remember that the effects of use and disuse, which he seems to see everywhere about him, are mixed up with plenty of apparent instances to the contrary.

He will smile, perhaps, when I tell him that Weismann cut off the tails of endless mice, and, breeding them together, found that tails invariably decorated the race as before.

I remember hearing Mr.Bernard Shaw comment on this experiment.

He was defending the Lamarckianism of Samuel Butler, who declared that our heredity was a kind of race-memory, a lapsed intelligence.

"Why," said Mr.Shaw, "did the mice continue to grow tails?
Because they never wanted to have them cut off." But men-folk are wont to shave off their beards because they want to have them off; and, amongst people more conservative in their habits than ourselves, such a custom may persist through numberless generations.