encountered by Darwin on the Galápagos were its iguanas.

He was particularly fascinated by the marine variety,

though it was far from flattering.

Darwin wrote that they were "hideous-looking creatures

"of a dirty, black colour, stupid and sluggish in their movements."

He did concede, however, that they were strong graceful swimmers.

Bearing in mind how much time they spent in the water,

Darwin was surprised to find that marine iguanas didn't eat fish.

He dissected several creatures

and found that their stomachs were full of seaweed.

Darwin noticed the water was too cold

for marine iguanas to stay in for a long time.

When they weren't feeding,

he observed them clinging to the rocks above the shoreline,

basking in the sun.

After the cold sea, these reptiles craved warmth.

Darwin was also interested in their terrestrial cousins,

although he wasn't complimentary about their appearance.

He called them "small, ugly animals with a singularly stupid appearance

"and lazy, half-torpid movements."

Today, they're still plentiful on some islands

but marine iguanas are particularly vulnerable

to changes in sea temperature,

which can have a dramatic effect on the availability of seaweed.