A major chamge in the evolutionary lineage leading to the chordates was the

I dont know"Robin Hood and the Scotchman"

Now bold Robin Hood to the north would go,

With valour and mickle might;

With sword by his side, which oft had been try’d,

To fight and recover his right.

The first that he met was a jolly stout Scot,

His servant he said he would be;

No quoth Robin Hood, it cannot be good.

For thou wilt prove false unto me.

Thou hast not been true to sire or cuz:

Nay marry, the Scot he said;

As true as your heart, I never will part,

Good master be not afraid.

But e’er I employ you, said bold Robin hood,

With you I must have a bout;

The Scotchman reply’d, let the battle be try’d,

For I know I will beat you out.

Thus saying, the contest did quickly begin,

Which lasted two hours and more;

The blows Sawney gave bold Robin so brave,

The battle soon made him give o’er.

Have mercy, thou Scotchman, bold Robin Hood cry’d,

Full dearly this boon have I bought;

We will both agree, and my man you shall be,

For a stouter I never have fought.

Then Sawney consented with Robin to go,

To be of his bowmen so gay;

Thus ended the fight, and with mickle delight

To Sherwood they hasted away.

Source: "Robin Hood and the Scotchman." The Robin Hood Garlands and Ballads. Ed. John Mathew Gutch, William Hone, Francis Douce, and Edward Francis Rimbault. London: John Russell Smith and Joseph Lilly, 1750. 392-93. Google Books. Web. 24 June 2011.

What is the structure of this poem?