Sink or swim? | Deep vision | Sacrificial candidate | Diverting days out Sir Christopher Ball telling people their future longevity is in their own hands and to stop blaming others ( Report, 20 May ) is akin to telling a drowning man to pull himself together and swim, without asking what were the circumstances that put him in the water in the first place. Life and longevity is a complex issue and simplistic solutions rarely hold water. Patrick Eckersley Woodbridge, Suffolk • A large eye does not give more detailed vision (acuity) but better vision in the dark. The owl is a familiar example. So a wide-eyed ichthyosaur ( Tentacles, pointy teeth and the T-rex of the sea: the Natural History Museum on beasts that once ruled the oceans, 21 May ) probably dived deep for its prey but had to contend with poor illumination. Extant abyssal animals such as giant squid have large eyes for the same reason. Julian Vincent Bath Continue reading...

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