Snakes

    No zoologist doubts that snakes evolved from reptiles. The ransformation of a four-legged into a legless reptile is said to 'have happened in several distinct varieties. Some snakes - pythons and boas - still have what seem to be relics of their hip bones inside them, and bulges on the outside where probably the legs once grew. 

The problem comes in explaining how the snake developed, through a very large number of chance mutations, its unique method of locomotion, which is distinctly unreptilian. It demands an increase in the number of certain vertebrae. Without this crucial variation, snakes could not wriggle about and move in the way they do. Yet it apparently happened in several independent groups of snakes in widely differing parts of the Earth and in different periods.  

Panel 15

Do we bear well enough? 

Darwin wrote: ‘Natural selection tends only to make each organic being as perfect as, or slightly more perfect than, the other inhabitants of the same country with which it has to struggle for existence.’

The trouble with the mammal ear is that, in terms of natural selection, it has nothing of enough significance to justify its enormously complex system having emerged by natural selection. Amphibians, reptiles, and birds, all of which have only one earbone, can perceive pitch and volume at least as well as mammals, and in some cases better.

The sole possible advantage is that mammals can hear to some extent stereophonically, while it is thought creatures with single earbones cannot do this quite so well. However, barn owls (and probably others as well) can locate prey in the dark by receiving sound stereophonically. The ears are slightly asymmetrically placed on the head. The sound emitted by the prey is received split seconds apart in oath ear. The brain decodes the information and works out coordinates. In the case of mammals, stereophony happens because our brains receive signals from both the outer and the inner ear, and the fractional delay in the sound impulses may enable us to estimate how far away a sound is coming from. In survival value, this might confer a minimal advantage in, for instance, spotting prey or escaping predators.

But even if this ability were proved (for doubts still remain), it is hard to see how the transitional forms leading up to it could have made the ear, in Darwin's words 'slightly more perfect'. The stereophonic effect can only work when the inner and outer ears have been fully displaced.

 Similarly, the snake's eating apparatus requires a large number of independent functions to evolve simultaneously: for instance, jawbone modifications, reshaping of the teeth, appearance of an extra row of teeth, special protection for the brain, modifications of the glottis, and appearance of powerful digestive juices. There is no trace of any transitional stages in the fossil record; nor has anyone ever seen, in the broods of four-legged lizards, a mutated monstrosity which might give a clue as to how a reptile can become both legless and perfectly adapted to a new way of locomotion and life.