Darwin, in the Origin, introduced the term ‘living fossil’ to describe primitive organisms which could be seen as relics from the past – members of groups once widespread but now greatly reduced and occurring only in very isolated and restricted environments (where ‘competition…will have been less severe than elsewhere’). Ginkgos, for example, were very widespread once – they were a dominant species of the Pacific Northwest before the great Spokane Flood fifteen million years ago – but are now restricted to a single species, found only in cultivation and in a small area of China. The most spectacular discovery of such a ‘living fossil’ in this century was that of a fish, the coelacanth Latimeria, in 1938; a more recent one, which shook the botanical world, was the discovery in 1994 of a gymnosperm long thought to be extinct, the Wollemi pine, in Australia. (I still hope, in some irrational, romantic part of myself, that a giant club moss or horsetail will turn up one day.)
But whereas ‘Latimeria chalumnae is a single species, only managing to survive in the special conditions off the Comoros, cycads (though no longer the dominant flora, as in the Mesozoic) still number more than two hundred species and still thrive in a wide variety of eco-climes, so they cannot really be called ‘living fossils.’
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The unexpected adaptation of crabs to coconut eating fascinated Darwin, who describes them in the Beagle:
I have before alluded to a crab which lives on the cocoa-nuts: it is very common on all parts of the dry land, and grows to a monstrous size: it is closely allied or identical with the Birgos latro.
The front pair of legs terminate in very strong and heavy pincers, and the last pair are fitted with others weaker and much narrower. It would at first be thought quite impossible for a crab to open a strong cocoa-nut covered with the husk; but Mr. Liesk assures me that he has repeatedly seen this effected. The crab begins by tearing the husk, fibre by fibre, and always from that end under which the three eye-holes are situated; when this is completed, the crab commences hammering with its heavy claws on one of the eye-holes till an opening is made. Then turning round its body, by the aid of its posterior and narrow pair of pincers, it extracts the white albuminous substance. I think this is as curious a case of instinct as ever I heard of, and likewise of adaptation in structure between two objects apparently so remote from each other in the scheme of nature, as a crab and a cocoa-nut tree…It has been stated by some authors that the Birgos crawls up the cocoa-nut trees for the purpose of stealing the nuts: I very much doubt the possibility of this; but with the Pandanus the task would be very much easier. I was told by Mr. Liesk that on these islands the Birgos lives only on the nuts which have fallen to the ground.
(In fact, coconut crabs do climb tall palm trees, and cut off the coconuts with their massive claws.)
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It used to be held that cycads were wind-pollinated, like ferns and conifers, though early authors (including Chamberlain) had occasionally been struck by the presence of certain insects in or near the male cones at the time of pollination.
In 1980, Knut Norstog and Dennis Stevenson, working at the Fairchild Tropical Garden in Miami, were struck by the failure of many introduced cycads there to produce fertile seeds, even though healthy male and female plants had been planted just a yard or two apart, whereas the native Zamia was quite fertile. They found that snout weevils would feed as larvae on the male Zamia cones, emerging as adults by boring through the microsporophylls, covered with pollen. Could this be the way in which the female cones were pollinated?
Stevenson and Norstog, along with other researchers (Karl Niklas, Priscilla Fawcett, and Andrew Vovides), have confirmed this hypothesis in great detail. They have observed that weevils feed and mate on the outside of the male cone and then enter it, continuing to feed not on the pollen, but on the bases of the microsporophylls. Their eggs are laid, and larvae hatched, inside the microsporophylls, and the adult weevils finally chew their way out through the tips of the sporophylls. Some of these weevils go to the female cones, which exude a special warmth and aroma when they are ready for pollination, but the weevils cannot feed here, since the female cones are toxic to the insects. Crawling into the female cone through narrow cracks, the weevils are divested of their pollen and, finding no reason to stay longer, they return to the male cones.
The cycad thus depends on the weevil for pollination, and the weevil on the cycad cones for warmth and shelter – neither can survive without the other. This intimate relationship of insects and cycads, this coevolution, is the most primitive pollination system known and probably goes right back to the Paleozoic, long before the evolution of flowering plants, with their insect-attracting scents and colors.
(A variety of insects can pollinate cycads, mostly beetles and weevils, though one species of Cycas is pollinated by a bee – giving the possibility, one likes to think, of a delicious cycad honey.)
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One cannot think of these beautiful adaptations without feeling how excellent cycads are, in their own way, and how meaningless it is to see them as ‘primitive’ or ‘lower’ plants, inferior in the scale of life to ‘higher’ flowering plants. We have this almost irresistible sense of a steady evolutionary advance or progress (culminating, of course, in nature’s ‘highest’ product – ourselves), but there is no evidence of any such tendency, any global progress or purpose, in nature itself. There is only, as Darwin himself insisted, adaptation to local conditions.
No one has written of our illusions about progress in nature with more wit and learning than Stephen Jay Gould, especially in his recent book, Full House: The Spread of Excellence from Plato to Darwin. They lead us, he writes, to a false iconography of the world, so that we see the Age of Ferns succeeded by the Age of Gymnosperms, succeeded by the present Age of Flowering Plants, as if the earlier forms of life had ceased to exist. But while many early species have been replaced, others continue to survive as highly successful, adaptable life forms, as with ferns and gymnosperms, which occupy every niche from rain forest to desert. If anything, we are really, Gould insists, in the Age of Bacteria – and have been for the last three billion years.
One cannot look at a single lineage, whether of horses or hominids, and come to any conclusions about evolution or progress, as Gould shows. We must look at the total picture of life on earth, of every species, and then we will see that it is not progress which characterizes nature but rather infinite novelty and diversity, an infinity of different adaptations and forms, none to be seen as ‘higher’ or ‘lower.’
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Darwin was the first to argue that dispersal of seeds by sea water might be an important means of their distribution, and made experiments to explore their ability to float and survive salt water. Many seeds, he found, had first to dry but then might float for remarkably long periods: dried hazelnuts, for example, floated for ninety days and afterwards germinated when planted. Comparing these time periods with the rates of ocean currents, Darwin thought that thousand-mile ocean journeys might be common for many seeds, even if they had no special flotation layer (like cycad seeds). ‘Plants with large seeds or fruit,’ he concluded, ‘generally have restricted ranges, fand] could hardly be transported by any other means.’