Nature is full of wonders. A human sings. Means, actually simple air is made to pass thro' a part in the body . Beautiful sound is produced. How? . By mentally meddling with that part, so many melodies appear in the voice/tone. All this is achieved just with the movement of air thro' that part of the body. No wonder?

Same air is passed thro' a part of the body of a bird or animal. Different sound. Again the mental meddling of that part of the body is not possible for those birds/animals , in general. Means, there is a thresh hold for the mind of the bird/animal and the human. Who set the thresh hold?

If evolution is the answer, how the evolution is going on so systematically that in general things maintain the levels of up gradation of calibers undisturbed and consistant?

For the up gradation also to occur, there should be something behind the change to take place from one grade to another. No ? - Every thing continues to be in its state of rest or of uniform motion unless it is compelled by an external force to change that state. - A scientific law by Newton. So what is the reason or force behind the change? Beyond our reasonable guess?