But Al Jahiz is describing natural selection.
No? He says "Environmental factors influence organisms to develop new characteristics to ensure survival, thus transforming into new species."
They dont' just develop these 'good' characteristics e.g. a giraffe stretching its neck and thus making it longer.
According to Darwin they start of with a multitude of characteristics, some good & some bad. Those with the 'good' characteristics i.e. those that match the environment optimally, are more likely to survive. Its an important difference, but its what diffentiated Darwin and made him who he is today.