An ostrich and a woodpecker are similar in a lot of ways, both birds descending from a common ancestor. But I don't think you'd confuse them.
My question still stands. If webkit is a bird and gecko is a reptile, then why are there (relatively) so few reptile browsers? Is it easier to embed engines from the webkit family, or are they popular because chrome and safari are popular?
Largely the former. The efforts to make Gecko easily embeddable, started some 15 years ago, have long floundered.