"In certain snakes there is a large actively functional right and a small comparatively useless left lung. The evolutionist would suggest as the meaning of this that the ancestors of snakes possessed, like normal air-breathing s, a pair of lungs—right and left—approximately equal in size, but that in correlation with the lengthening of the body as it assumed the typical snake form the left lung had undergone a gradual reduction in size. He would point out that in some snakes, such as the , a still further stage has been reached in which the left lung has disappeared altogether, leaving only a single—the originally right—lung."