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that the remains were preserved of the Ichthyosauri and Plesiosauri, which appear to have represented the Cetacea during the secondary geological period.
SKELETON OF A PLESIOSAURUS,
As another example let us consider the origin of wings, as they exist in birds. Here we find in fact an arm, the bones of the hand of which are atrophied and reduced in number, as compared with those of most other Vertebrates. Now, if the wing took its origin from a terrestrial or subaërial organ, this abortion of the bones could hardly have been serviceable—hardly have preserved individuals in the struggle for life. If it arose from an aquatic organ, like
the wing of the penguin, we have then a singular divergence from the ordinary vertebrate fin-limb. In the ichthyosaurus, in the plesiosaurus, in the whales, in the porpoises, in the seals, and in others, we have shortening of the bones, but no reduction in the number either of the fingers or of their joints, which are, on the contrary, multiplied in Cetacea and the ichthyosaurus. And even in the
turtles we have eight carpal bones and five digits, while no finger has less than two phalanges. It is difficult, then, to believe that the Avian limb was developed in any other way than by a comparatively sudden modification of a marked and important kind.
How, once more, can we conceive the peculiar actions of the tendrils of some climbing plants to have been produced by minute modifications ? These, according to Mr. Darwin, oscillate till they touch an object, and then embrace it. It is stated by that observer, “that a thread weighing no more than the thirty-second of a grain, if placed on the tendril of the Passiflora gracilis, will cause it to bend; and merely to touch the tendril with a twig causes it to bend; but if the twig is at once removed, the tendril soon straightens itself. But the contact of other tendrils of the plant, or of the falling of drops of rain, do not produce these
Some of the zoological and anatomical discoveries of late years tend rather to diminish than to augment the evidence in favour of minute and gradual modification. Thus all naturalists now admit that certain animals, which were at one time supposed to be connecting links between groups, belong altogether to one group, and not at all to the other. For example, the aye-aye (Chiromys Madagas
1 Quarterly Journal of Science, April 1866, pp. 257–8. 2 “Habit and Intelligence,” vol. i. p. 178.
3 This animal belongs to the order Primates, which includes man, the apes, and the lemurs. The lemurs are the lowest kinds of the order, and differ much from the apes. They have their head-quarters in the Island of Madagascar. The aye-aye is a lemur, but it differs singularly from all its congeners, and still more from all apes. In its dentition it strongly approximates to the rodent (rat, squirrel, and guinea pig) order, as it has two cutting teeth above, and two below, growing from permanent pulps, and in the adult condition has no canines.
cariensis) was till lately considered to be allied to the squirrels, and was often classed with them in the rodent order, principally on account of its dentition; at the same time that its affinities to the lemurs and apes were admitted. The thorough investigation into its anatomy which has now been made demonstrates that it has no more essential affinity to rodents than any other lemurine creature has.
Bats were, by the earliest observers, naturally supposed to have a close relationship to birds, and cetaceans to fishes. It is almost superfluous to observe that all now agree that these mammals make not even an approach to either one or other of the two inferior classes.
In the same way it has been till recently supposed that those extinct flying saurians, the pterodactyles, had an affinity with birds more marked than any other known animals. Now, however, as has been already said, it is contended that not only had they no such close affinity, but that other extinct reptiles had a far closer one.
The amphibia (i.e. frogs, toads, and efts) were long considered (and are so still by some) to be reptiles, showing an affinity to fishes. It now appears that they form with the latter one great group-the ichthyopsida of Professor Huxley—which differs widely from reptiles ; while its two component classes (fishes and amphibians) are difficult to separate from each other in a thoroughly satisfactory manner.
If we admit the hypothesis of gradual and minute modification, the succession of organisms on this planet must have been a progress from the more general to the more special, and no doubt this has been the case in the majority of instances. Yet it cannot be denied that some of the most recently formed fossils show a structure singularly more generalized than any exhibited by older allied forms; while others are more specialized than are any similar creatures of the existing creation.
A notable example of the former circumstance is offered by macrauchenia, a hoofed animal, which was at first supposed to be a kind of great llama (whence its name)the llama being a ruminant, which, like all the rest, has two toes to each foot. Now hoofed animals are divisible into two very distinct series, according as the number of functional toes on each hind foot is odd or even. And many other characters are found to go with this obvious
Even the very earliest Ungulata show this distinction, which is completely developed and marked even in the Eocene palæotherium and anoplotherium found in Paris by Cuvier. The former of these has the toes odd (perissodactyle), the other has them even (artiodactyle).
Now, the macrauchenia, from the first relics of it which were found, was thought to belong, as has been said, to
the even-toed division. Subsequent discoveries, however, seemed to give it an equal claim to rank amongst the perissodactyle forms. Others again inclined the balance of probability towards the artiodactyle. Finally, it appears that this very recently extinct beast presents a highly generalized type of structure, uniting in one organic form both artiodactyle and perissodactyle characters, and that in a manner not similarly found in any other known creature living, or fossil. At the same time the differentiation of artiodactyle and perissodactyle forms existed as long ago as in the period of the Eocene ungulata, and even in a marked degree, as has been before observed.
Again, no armadillo now living presents nearly so remarkable a speciality of structure as was possessed by the extinct glyptodon. In that singular animal the spinal column had most of its joints fused together, forming a rigid cylindrical rod, a modification, as far as yet known, absolutely peculiar to it.
In a similar way the extinct machairodus, or sabre-toothed tiger, is characterized by a more highly differentiated and specially carnivorous dentition than is shown by any predacious beast of the present day. The specialization is of this kind. The grinding teeth (or molars) of beasts are divided into premolars and true molars. The premolars are molars which have deciduous vertical predecessors (or milk teeth), and any which are in front of such, i.e. between such and the canine tooth. The true molars are those placed behind the molars having deciduous vertical pre: decessors. Now, as a dentition becomes more distinctly carnivorous, so the hindermost molars and the foremost premolars disappear. In the existing cats this process is carried so far that in the upper jaw only one true molar is