他用老式打字机写作的习惯是对过去的一种怀旧。

His habit of writing on a vintage typewriter is a ___throwback to the past.

这只蝴蝶翅膀上的花纹是一种返祖现象，与它的远古祖先非常相似。

The pattern on this butterfly's wings is a ___throwback to its ancient ancestors.