Since Darwin, biologists have been firmly convinced that nature works without plan or meaning, pursuing no aim by the direct road of design. But today we see that this conviction is a fatal error. Why should evolution, exactly as Darwin knew it and described it, be planless and irrational? Do not aircraft design engineers work, at precisely that point where specific calculations and plans give out, according to the same principle of evolution, when they test the serviceability of a great number of statistically determined forms in the wind tunnel, in order to choose the one that functions best? Can we say that there is no process of natural selection when nuclear physicists, through thousands of computer operations, try to find out which materials, in which combinations and with what structural form, are best suited to the building of an atomic reactor? They also practise no designed adaptation, but work by the principle of selection. But it would never occur to anyone to call their method planless and irrational.