To emphasize how odd the idea of a creator sticking rigidly to 'themes' is, reflect that any sensible human designer is quite happy to borrow an idea from one of his inventions, if it would benefit another. Maybe there is a 'theme' of aircraft design, which is separate from the 'theme' of train design. But a component of a plane, say an improved design for the reading lights above the seats, might as well be borrowed for use in trains. Why should it not, if it serves the same purpose in both? When motor cars were first invented, the name 'horseless carriage' tells us where some of the inspiration came from. But horse-drawn vehicles don't need steering wheels - you use reins to steer horses - so the steering wheel had to have another source. I don't know where it came from, but I suspect that it was borrowed from a completely different technology, that of the boat. Before being superseded by the steering wheel, which was introduced around the end of the nineteenth century, the original steering device of the car was the tiller, also borrowed from boats, but moved from the rear to the front of the vehicle.
If feathers are a good idea within the bird 'theme', such that every single bird, without exception, has them whether it flies or not, why do literally no mammals have them? Why would the designer not borrow that ingenious invention, the feather, for at least one bat? The evolutionist's answer is clear. All birds have inherited their feathers from their shared ancestor, which had feathers. No mammal is descended from that ancestor. It's as simple as that.* The tree of resemblances is a family tree. It is the same kind of story for every branch and every sub-branch and every sub-sub-branch of the tree of life.
Now we come to an interesting point. There are plenty of beautiful examples where it looks, superficially, as though ideas might have been 'borrowed' from one part of the tree and grafted on to another, like an apple variety grafted on to a stock. A dolphin, which is a small whale, looks superficially like various kinds of large fish. One of these fish, the dorado (Coryph&na hippuris) is even sometimes called a 'dolphin'. Dorados and true dolphins have the same streamlined shape, suited to their similar ways of life as fast hunters near the surface of the sea. But their swimming technique, though superficially similar, was not borrowed from one by the other, as you can quickly see if you look at the details. Although both derive their speed mostly from the tail, the dorado, like all fish, moves its tail from side to side. But the true dolphin betrays its mammal history by beating its tail up and down. The side-to-side wave travelling down the ancestral fish backbone has been inherited by lizards and snakes, which could almost be said to 'swim' on land. Contrast that with a galloping horse or cheetah. The speed comes from bending of the spine, as it does with fish and snakes; but in mammals the spine bends up and down, not side to side. It is an interesting question how the transition was made in the ancestry of mammals. Maybe there was an intermediate stage, which hardly bent its spine at all, in either direction, like a frog. On the other hand, crocodiles are capable of galloping (frighteningly fast) as well as using the lizard-like gait more conventional among reptiles. The ancestors of mammals were nothing like crocodiles, but maybe crocodiles show us how an intermediate ancestor might have combined the two gaits.
Anyway, the ancestors of whales and dolphins were fully paid-up land mammals, who surely galloped across the prairies, deserts or tundras with an up-and-down flexion of the spine. And when they returned to the sea, they retained their ancestral up-and-down spinal motion. If snakes 'swim' on land, dolphins 'gallop' through the sea! Accordingly, the fluke of a dolphin may look superficially like the forked tail of a dorado, but it is set horizontally, whereas the dorado's tail fins are aligned in the vertical plane. There are numerous other respects in which the dolphin's history is written all over it, and I shall come to them in the chapter of that title.
There are other examples where the superficial resemblance is so great that it seems quite hard to reject the 'borrowing' hypothesis, but a closer inspection shows that we must. Animals can look so alike that you feel they must be related. But it then turns out that the similarities, though impressive, are outnumbered by the differences when you look at the whole body. 'Pill bugs' (see over) are familiar little creatures, with lots of legs, who habitually roll up into a protective ball, like armadillos. Indeed, this may be the origin of the Latin name Armadillidium. That is the name of one kind of 'pill bug', which is a crustacean, a woodlouse, related to shrimps but living on land - where it betrays its recent aquatic ancestry by breathing with gills, which have to be kept moist. But the point of the story is that there is a completely different kind of 'pill bug' which is not a crustacean at all but a millipede. When you see them rolled up, you'd think they were almost identical. Yet one is a modified woodlouse, while the other is a modified (modified in the same direction) millipede. If you unroll them and look carefully, you will immediately see at least one important difference. The pill millipede has two pairs of legs on most segments, the pill woodlouse only one. Isn't it beautiful, all this endless modification? A more detailed examination will show that, in hundreds of respects, the pill millipede really does resemble a more conventional millipede. The resemblance to a woodlouse is superficial - convergent.
Almost any zoologist who was not a specialist would say that the skull on the opposite page belongs to a dog. The specialist would discover that it isn't actually a dog skull by noting the two prominent holes in the roof of the mouth. These are tell-tale signs of marsupials, the large group of mammals nowadays found mostly in Australia. It is in fact the skull of Thylacinus, the 'Tasmanian wolf'. Thylacines and true dogs (for example dingos, with which they competed in Australia and Tasmania) have converged on a very similar skull because they have (had, alas, in the case of the unfortunate thylacine) a similar lifestyle.
I have already mentioned the magnificent marsupial mammal fauna of Australia, in the chapter on the geographical distribution of animals. The relevant point for this chapter is the repeated convergences between these marsupials and a great variety of opposite numbers among the 'placental' (i.e. non-marsupial) mammals, which dominate the rest of the world. Though far from identical, even in superficial characteristics, each marsupial in the illustration overleaf is sufficiently similar to its placental equivalent - that is, the placental that most closely practises the same 'trade' - to impress us, but certainly not sufficiently similar to suggest 'borrowing' by a creator.
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