It would make the average cave look enviably well-appointed, and when we knock on the back door there is an explosion of fur through the broken kitchen window as three semi-feral cats scramble for the safety of the garden. Unfortunately they leave their smell behind them - a rank stench that claws its way up somewhere behind the sinuses and refuses to come down again for several hours.It doesn't seem to disturb the occupier, a mild and slightly fretful figure who tells Dermot that the last litter of kittens has already been taken away by a lady from Help the Aged, and who finally agrees to trap the female cat and surrender her for spaying, so that there won't be another. The large boxes of cat food on the filthy concrete floor are evidence of the odd symbiosis that often exists between semi-feral cats and lonely humans - a companionship the RSPCA doesn't like to break up, provided both parties are able to cope. Euthanasia is a solution of last resort, and only turned to when a cat is so old or infirm that it is impossible to rehouse it.When the female is eventually caught, Dermot will take her the charity's Putney clinic, and find the funds to arrange for her operation. Recuperating in one of the cat wards there, we find another patient, wary and cautious at the back of her cage, a patch of suede and a stitched incision on one rump where her fur has been shaved off Around her other strays are also recovering. The problem, as Dermot explains, is that without comprehensive neutering, supply massively exceeds demand. Cats may be Britain's most popular pet, but the vigour of their procreation ensures that there will always be a surplus.Our final call of the day is to the RSPCA's Southall cattery - a kind of feline transit camp where post-operative strays can recuperate and members of the public can come to satisfy their desire for fully MOT'd pets (they all come microchipped, vaccinated and neutered).

It is spotlessly clean and, for the moment at least, surprisingly tranquil. But the peace is apparently deceptive: "This is the lull before the storm," explains Beverley Leary, who runs the centre. Numbers will rocket in March and April when new litters start to be born, and with at least 20 cats arriving each week from the RSPCA's two London clinics the matter of finding homes will become urgent.Kittens provide few problems, since they are the crack cocaine of cuteness - one hit and most people are hooked. (I almost succumb myself in the face of two five-week-old tabbies, but am snapped back to common sense by the vivid sense memory of picking cat turds out of gravel with a polythene bag wrapped round my hand.) Further down the room, though, we find the very opposite end of the cat desirability spectrum - an abandoned Persian of quite breathtaking ugliness. She has been "dematted" - her uncombable fur cut off - so that she looks like a French collaboratrice just prior to tarring, and her muzzle is so short that the tongue protrudes at all moments of repose.

The RSPCA managed to rehome a staggering 46,786 cats in 1999, but I think they may have met their match.If you are the sort of person who likes to give their compassion an arduous workout, the RSPCA have a cat for you. If not, then I can assure you that - however you look at it - she represents a powerful case for the advantages of neutering. More from Thomas Sutcliffe. A new scientific picture of the world, showing how man has left an indelible mark on the planet that is visible from space, was published yesterday by scientists who warned that the Earth was undergoing an unprecedented transformation. A new scientific picture of the world, showing how man has left an indelible mark on the planet that is visible from space, was published yesterday by scientists who warned that the Earth was undergoing an unprecedented transformation. The map, compiled from satellite images, shows almost a quarter of the Earth's surface has been entirely transformed, either by being covered overby roads and buildings or ploughed up for crops. Another quarter has been exploited to a lesser degree, but in a way that has completely altered its natural state.A rapidly growing human population, rising economic expectations, continual decline in natural resources and increasing pollution by industrialised countries are leading to a crisis of epic proportions.This stark warning is contained in a new atlas of the world showing how humans have had a devastating impact on the natural environment. The report, compiled by the American Association for the Advancement of Science (AAAS), was published on the opening day of its annual meeting in San Francisco."We have become a force of nature comparable to volcanoes or to cyclical variations in the Earth's orbit," the report warns.