My grammar book says when we want to talk about a race or a whole class we use the.
I think I agree, Subhajit.
When speaking generically, though, the plural and the singular preceded by the indefinite article are preferred, in that order. The singular will be more usual when reference is made to any individual member of the group.
- Giraffes are the tallest animals on earth.
- A giraffe can eat 30 kg of leaves per day.
- Inland taipans are extremely dangerous.
- An inland taipan can kill as many as 100 people with its venom.
In your sentences (1) and (2) above, I think "the" works well.