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NA TURE 



[September 2, 1Q09 



must also eventually have hindered rather than aided the 

 capture and eating of prey. The curious gradual elongation 

 of the face in the Oligocene and Miocene Mastodons, which 

 has lately been described by Dr. Andrews, can only be 

 regarded as another illustration of the same phenomenon. 

 In successive generations of these animals the limbs seem 

 to have grown continually longer, while the neck remained 

 short, so that the head necessarily became more and more 

 elongated to crop the vegetation on the ground. A \\m\\ 

 of mechanical inefficiency was eventually reached, and then 

 there survived only those members of the group in which 

 the attenuated mandible became shortened up, leaving the 

 modified face to act as a " proboscis." The elephants thus 

 arose as a kind of after-thought from a group of quadrupeds 

 that were rapidly approaching their doom. 



The end of real progress in a developing race of back- 

 boned animals is also often marked by the loss of the 

 teeth. A regular and complete set of teeth is always present 

 at the commencement, but it frequently begins to lack 

 successors in animals which have reached the limit of their 

 evolution, and then it soon disappears. Tortoises, for in- 

 stance, have been toothless since the Triassic period, when 

 they had assumed all their essential features ; and birds 

 have been toothless since the end of Cretaceous times. The 

 monotreme mammals of Australasia, which are really a 

 survival from the Jurassic period, are also toothless. Some 

 of the latest Ichthyosaurs and Pterodactyls were almost or 

 quite toothless ; and I have seen a jaw of an Upper 

 Cretaceous carnivorous Dinosaur (Genyodectes) from Pata- 

 gonia so completely destitute of successional teeth that it 

 seems likely some of these land reptiles nearly arrived at 

 the same condition. 



Among fishes there is often observable still another sign 

 of racial old age — namely, their degeneration into eel-shaped 

 forms. The Dipnoan fishes afford a striking illustration, 

 beginning with the normally shaped Dipterus in the Middle 

 Devonian, and ending in the long-bodied Lepidosiren and 

 Protopterus of the present day. The PaIa;ozoic Acanthodian 

 sharks, as they are traced upwards from their beginning 

 in the Lower Devonian to their end in the Permian, also 

 acquire a remarkable elongation of the body and a fringe- 

 like extension of the fins. ."Vniong higher fishes, too, there 

 are numerous instances of the same phenomenon, but in 

 most of these the ancestors still remain undiscovered, and 

 it would thus be tedious to discuss them. 



p-inally, in connection with these obvious symptoms of 

 old age in races, it is interesting to refer to a few strange 

 cases of the rapid disappearance of whole orders of animals, 

 which had a practically world-wide distribution at the time 

 when the end came. Local extinction, or the disappear- 

 ance of a group of restricted geographical range, mav be 

 explained by accidents of many kinds ; but contemporaneous 

 universal extinction of widely spread groups, w'hich are 

 apparently not affected by any new competitors, is not so 

 .•=^sily understood. The Dinosaurs, for instance, are known 

 to have lived in nearly all lands until the close of the 

 Cretaceous period ; and, except perhaps in Patagonia, they 

 were always accompanied until the end bv a tvpically 

 Mesozoic fauna. Their remains are abundant in the 

 Wealden formation of Western Europe, the deposit of a 

 river which must have drained a great continent at the 

 beginning of the Cretaceous period ; they have also been 

 found in a corresponding formation which covers a large 

 area in the State of Bahia, in Brazil. They occur in great 

 numbers in the freshwater L'pper Cret.aceous Laramie de- 

 posits of Western North America, and also in a similar 

 formation of equally late date in Transylvania, South-east 

 Europe. In only two of these regions (South-east England 

 and West North .America) have any traces of mammals 

 been found, and they are extremely rare fragments of 

 animals as small as rats ; so there is no reason to suppose 

 that the Dinosaurs suffered in the least from any struggle 

 with warm-blooded competitors. Even in Patagonia, where 

 the associated mammal-remains belong to slightly larger 

 and more modern animals, these fossils are also rare, and 

 there is nothing to suggest competition. The race of 

 Dinosaurs seems, therefore, to have died a natural death. 

 The same may be said of the marine reptiles of the orders 

 Ichthyosauria, Plesiosauria, and Mosasauria. Thev had a 

 practically world-wide distribution in the seas of the 

 Cretaceous period, and the Mosasauria especyallv must have 

 been extremely abundant and flourishing. Nevertheless, at 

 NO. 2079, VOL. 81] 



the end of Cretaceous times they disappeared everywhere, 

 and there was absolutely nothing to take their place until 

 the latter part of the Eocene period, when whales and 

 porpoises began to play e.xactly the same part. So far as 

 we know, the higher race never even came in contact with 

 the lower race ; the marine mammals found the seas vacant, 

 except for a few turtles and for one curious Rhyncho- 

 cephalian reptile (Champsosaurus), which did not long sur- 

 vive. Another illustration of the same phenomenon is 

 probably afforded by the primitive Carnivora (the so-called 

 Sparassodonta), which were numerous in South .America in 

 the Lower Tertiary periods. They were animals with a 

 brain as small as that of the thylacines and dasyures which 

 now live in Tasmania. They appear to have died out com- 

 pletely before they were" replaced by the cats, sabre-toothed 

 tigers, and dogs, which came down south from North 

 America over the newlv emerged isthmus of Panama at the 

 close of the Pliocene period. At least, the remains of these 

 old carnivores and their immigrant successors have never 

 yet been found associated in any geological formation. 



These various considerations lead me to think that there* 

 is also deep significance in the tendency towards fixity in 

 the number and regularity (or symmetry) in the arrange- 

 ment of their multiple parts, which we frequently observe 

 in groups of animals as we trace them from their origin to 

 their prime. It is well known that in certain of the highest 

 and latest types of bony fishes the vertebra; and fin-rays 

 are reduced to a fixed and practically invariable number 

 for each family or genus, whereas there is no such fixity 

 in the lower and earlier groups. In the earliest known 

 Pycnodont fishes from the Lower Lias (MesodonJ the grind- 

 ing teeth form an irregular cluster, while in most of the 

 higher and later genera they are arranged in definite 

 regular rows in a symmetrical manner. Many of the lower 

 backboned animals have teeth with several cusps, and in 

 some genera the number of teeth seems to be constant ; but 

 in the geological history of the successive classes the tooth- 

 cusps never became fixed individual entities, readily trace- 

 able throughout whole groups, until the highest or mam- 

 malian grade had been attained. Moreover, it is only in 

 the same latest grade or class that the teeth themselves can 

 be treated as definite units, always the same in number 

 (fortv-four), except when modified by degeneration or 

 special adaptation. In the earlier and lower land animals 

 the number of vertebras in the neck depends on the extent 

 of this part, whereas in the mammal it is almost invariably 

 seven, whatever the total length may be. Curiously con- 

 stant, too, in the modern even-toed hoofed mammals is the 

 number of nineteen vertebrae between the neck and the 

 sacrum. 



I am therefore still inclined to believe that the comparison 

 of vital processes with certain purely physical phenomena 

 is not altogether fanciful. Changes towards advancement 

 and fixity which are so determinate in direction, and 

 changes towards extinction which are so continually re- 

 peated, seem to denote some inherent property in living 

 things, which is as definite as that of crystallisation in 

 inorganic substances. The regular course of these changes 

 is merely hindered and modified by a succession of checks 

 from the environment and Natural Selection. Each separate 

 chain of life, indeed, bears a striking resemblance to a 

 crystal of some inorganic substance which has been dis- 

 turbed by impurities during its growth, and has thus been 

 fashioned with unequal faces, or even turned partly into a 

 mere concretion. In the case of a crystal the inherent 

 forces act solely on molecules of the crystalline substance 

 itself, collecting them and striving, even in a disturbing 

 environment, to arrange them in a fixed geometrical shape. 

 In the case of a chain of life (or organic phylum) we may 

 regard each successive animal as a temporary excrescence 

 of colloid substance round the equally colloid germ-plasm 

 which persists continuously from generation to generation. 

 The inherent forces of this germ-plasm, therefore, act upon 

 a consecutive series of excrescences (or animal bodies), 

 struggling not for geometrically arranged boundaries, but 

 towards various other symmetries, and a fixity in number 

 of multiple parts. When the extreme has been reached, 

 activities cease, and sooner or later the race is dead. 



Such are some of the most important general results to 

 which the study of fossils has led during recent years ; and 

 thev are conclusions which every new discovery appears to 

 make more certain. When we turn to details, however, it 



