The sad thing is that when Ben and Bebe came to London in 1955 the Northern was probably the commoner White subspecies. If London (or more realistically) Whipsnade and maybe half a dozen other places in Europe and North America had set up breeding groups in the next decade it's hard not to imagine that the animal would now be quite safe, regardless of the problems that would drive it to extinction in Africa.
True enough. If Whipsnade had had even a small founder group of this species. e.g. 3.3 they could probably have emulated the success they have had with the Southern whites.
