【英语时差8,16】如何界定性别？2

 Another sexual difference among mammals that doesn’t hold with other animals is size. In most mammals the male is larger than the female, but in most other animals where there are two distinct sexes, the female is the larger one. Today scientists define female as the ability to produce eggs and male as the ability to produce sperm¹. But this is a relatively² new distinction since until the 19th century no one knew whether mammals even produced eggs. And it wasn’t until the 20th century that we were able to see the similarities between mammalian eggs and other eggs. So even though we talk about male and female in most species of animals, the two terms can have extremely different meanings.

1 sperm   

n.精子，精液

参考例句：

• Only one sperm fertilises an egg.只有一个精子使卵子受精。
• In human reproduction,one female egg is usually fertilized by one sperm.在人体生殖过程中，一个精子使一个卵子受精。

2 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。