Clifford D. Simak 
Our Children’s Children [EPUB ebook] 

An adventure of the near future – brought on by a catastrophe in the far future. On a summer’s day like any other, holes appeared in the air and people from nowhere walked through them into our world. They came, and they kept coming, until they numbered in the millions. They said they came from the future – they were our children’s children. They said there was trouble "up there". They knew they were a terrible burden on our economy, and all, but – well, we couldn’t just let them starve, could we? Then more facts became apparent.The "holes in the air" were time tunnels, one-way tunnels from the future. The trouble up there was in the form of alien creatures – ravening beasts with teeth, claws, and tentacles, that reproduced like bacteria and were intelligent.They were utterly uncontrollable and so our children’s children fled through the time tunnels, which, they claimed, were securely guarded. The beasts, whatever or whoever they were, couldn’t get through. So they claimed. But then somebody up there slipped and the beasts were abroad.