The industries of the South-east and the docks themselves declined during the 60s and 70s, reducing the amount of river traffic and pollution. Animal life, they said, could no longer survive in any measurable form in waters so bereft of oxygen. But even as the Thames reached its nadir, things began to change. In 1957, scientists declared the river Thames biologically dead, killed by the pollution that was a consequence of industrialisation and urban growth. Counting back, that’s two from the first generation, four from the second etc.CARMEL SWANNNEWHAVEN, EAST SUSSEX. As for the argument that cannabis leads on to other drugs, it’s a fair bet that it was alcohol that started the downwards spiral .STEPHEN DAVEYBLACKWOOD, CAERPHILLY Equal ancestors Sir: The unknown Mongolian may have 800,000 times more descendants than the average male (“Giants of the gene pool, 19 January), but (allowing for intermarriage, advances in cloning technology and parthenogenesis) we all have the same number of ancestors.

Staff in post offices, banks, etc now ask that headwear (crash helmets, baseball caps, hoods, etc) that conceals the face from security cameras be removed. Why not the veil/burka which has the same effect? The wearer may not be a Muslim woman.MARCUS CATLINGSKIPTON, NORTH YORKSHIRE Alcohol abuse Sir: How long before people realise that alcohol is more of a problem in society than cannabis ever will be (reports, 19 January)? Debating on how the latter should be classified is a waste of everyone’s time and effort. For countries such as the US, UK, France and Germany to deny Iran this right based on some notion of a hidden agenda is a poor argument. Which countries that have a nuclear capability have not developed it in secrecy?I predict that Iran will develop a successful nuclear programme for its energy needs. The French equivalent of “Yeah but no but” is in fact “Mais oui mais non”, and has been said for decades by French intellectuals, generally accompanied with a Gallic shrug, as a way of hogging the conversation while they think of what to say next.DR MATTHEW COBB FACULTY OF LIFE SCIENCESUNIVERSITY OF MANCHESTER Orchid origins Sir: Your report of the conviction of orchid smuggler Dr Sian Lim was accompanied by a photograph of Paphiopedilum sanderianum, not rothschildianum (“Scientist jailed for smuggling Malaysia’s rarest orchids”, 18 January).Both were introduced by my great grandfather, Frederick Sander, in 1885 and 1888 respectively; at that time the importation from the wild of these beautiful species was not only not illegal, but had royal approval!DR PETER SANDERLYME REGIS, DORSET Hidden faces Sir: Dr Rashed Akhtar challenges anyone to provide examples of women wearing the veil (burka) being associated with any criminal activity (Letters, 20 January).The burka has certainly been worn in a post office raid not far from here, the sex of the persons wearing them is unknown, though thought to be male The effect on the staff was terror.

What better time to inaugurate United Kingdom Day?T J HONEYBONEWHEATLEY, YORKSHIRE French ambivalence Sir: A franchophile like Miles Kington (19 January) should know better. Better to engage with Iran now and let it develop its nuclear programme under the guidance and controls of the IAEA.CARLOS LAVICTOIREWORTH, WEST SUSSEX UK day Sir: Most of the suggestions (Letters, 18 January) for a British National Day so far seem to concentrate on celebrating military victories over our European neighbours or subjugating a quarter of the globe to British rule.What’s wrong with commemorating the anniversary of the creation of the United Kingdom? The 300th anniversary of this event falls on 1 May 2007. After Iraq, there would be little political will for a military solution. Never mind, let us just enjoy the chance to do our shopping in peace.DAVID STOCKSGEORGINA STOCKSTEFFONT, WILTSHIRE Guidance for Iran’s nuclear programme Sir: According to the Nuclear Non-Proliferation Treaty, Iran has the legitimate right to research and implement a nuclear solution for its energy needs. Whenever we go shopping, there are hordes of people with clipboards asking other people to answer questionnaires. We sail through, confident that nobody will ask us anything.This attitude causes us to wonder why it is that we are always being told that we have an ageing population and that we are in the majority, yet the majority is not being canvassed? Presumably we do not have a big enough disposable income, nor are our opinions worth any consideration. We, both in our seventies, have been revelling in our invisibility.