THOSE who criticise scientists for playing God and call for checks on the extent to which their experiments should be allowed to tinker with - or threaten - Creation are easily condemned as reactionary and small-minded. Yet, the need for ethical control of scientific experimentation is becoming increasingly evident. Scientists have known for many years how to clone animal embryos and create identical litters of two or more at will. But while they have had the theoretical knowhow to use these techniques with people, it is only now that anyone has admitted to cloning human identical twins. What Dr Jerry Hall of George Washington University claims to have done raises serious ethical questions. They cannot be dismissed as the quixotic ramblings of flat-earthers determined to hold back the advance of scientific knowledge. If nature produces identical twins they are chance creations which add to the richness and diversity of the human race. If man begins to do it, it takes little imagination to understand that it will reduce the diversity of the human race. It will also offer opportunities for mass manipulation and control of society and, no doubt, provide new living material for further scientific experiment. The spectre of vivisection has already been raised by the grotesque suggestion that parents could keep a second embryo on ice, to be brought on stream as a source of spare parts should the sibling require a transplant later in its life. The further uses - or abuses - to which such a bank of human embryos could be put, particularly once the physical and mental attributes and potential of the adult siblings have been thoroughly mapped out, are cause for concern. The more extreme have already been explored in such apocalyptic fiction as Aldous Huxley's Brave New World or Ira Levin's The Boys from Brazil. That they could now be closer to being repeated in reality makes all the more urgent international discussion of the means to monitor, control and possibly ban further experimentation.