Amphibians and reptiles generally fertilize their eggs and abandon them to the elements, while birds and mammals raise their young, and so are bound closer to life from birth.

• 两栖类和爬行类一般是卵生的，产卵后通常直接弃之外界环境，不得不屈从于大自然的影响；而鸟类和哺乳动物抚养他们的后代，因此从诞生起就与生命更紧密地联系在一起。