NATURE 



[May 3, 1894 



Through the deep clear water I saw the heap of ballast, which 

 had been undermined and was settling down into the depths, 

 being already far below the level of the surrounding sand. 

 When the last of the timbers shall have yielded to the axe and 

 the waves, the sand will soon level up the hole caused by the 

 scour round the obstructing mass, and this heap of Scandinavian 

 boulders will lie buried in the sand till some exceptional storm 

 shall shift the lianks, and expose them again, and perhaps 

 transport them along the shore. 



Had this vessel been thrown on a hard rocky shore instead, 

 the ballast would have started at once with the other boulders 

 on the shore, and been scattered, according to size and form, 

 along the coast. As it was, however, these have got buried 

 deep in sand, and preserved till, perhaps, the habit of using 

 such boulders for ballast shall have been given up, and then, 

 washed out by the accidents of weather, of coast destruction, 

 and of shifting sand, they will appear among the fallen frag- 

 ments of a boulder clay cliff, and be appealed to in proof of its 

 origin. 



How many ships with Scandinavian ballast have been wrecked 

 along our eastern coast ever since the time of the Vikings ? 

 How many hundred tons of such boulders are still travelling 

 round our shores ? 



Another source of error I observed, this spring, along the Nor- 

 folk coast near Lowestoft. A perpetually changing underclifT 

 is formed by slips along the base of the cliffs. When the wind 

 blows hard from the northeast, the shingle is thrown up against 

 these broken masses, and much of it rests on the ledges and 

 terraces at various heights above the sea. Shells are tossed up 

 still higher, and gravel and sand from the upper part of the 

 cliff slide and pour down, and find a resting-place here and 

 there on its irregular face. When all these various processes 

 are seen going on around, and the easily identified patches of 

 recent shingle and shells or ancient sand and gravel can be 

 observed, with their track from above, or their obvious equivalent 

 below, there is not likely to be any difficulty. But when, in 

 subsequent storms, landslips have covered these diversified 

 patches with the samples of the various deposits that make up 

 the cliffs on that changeful coast, the interpretation is not always 

 soclear. Here we find in the boulder clay a mass of gravel 

 with shells derived largely from the cr.ag, there a streak of 

 shelly sand tossed up from the recent shore, and covered by a 

 slipped mass of boulder clay. Large boulders from the glacial 

 drift lie side by side with others that have travelled along the 

 shore from buildings or from wrecks ; the explanation of the 

 companionship being here and there given by the occurrence 

 of a tobacco-pipe or the thick end of a glass bottle. 



These are some of the more recent sources of error in our 

 attempts to learn the history of deposits from their boulders. 

 We must remember also in East Anglia that much of the drift 

 is derived from cretaceous boulder-bearing beds, and where 

 these appear sporadically in the drift they cannot be distinguished 

 from others which the ice has received firsthand from the parent 

 rock — except when clear traces of glaciation have been pre- 

 served. It is not enough, therefore, to record that a boulder 

 has been found on such a shore, or even in such a cliff, unless 

 the observer has been careful to note the exact conditions and 

 the surroundings of each find. 



Cambridge. T. McKenny Hughes. 



On the Tritub:rcular Theory. 



In a brilliant address, read last year before the American A sso- 

 ciation for the Advancement of Science,' Prof. II. F. Osborn has 

 brought together and laid before us the latest results of American 

 research lo which Mammalian Paleontology owes so much. 

 Necessarily much space is given to the exposition of the theory 

 of the devclopmeni of the cusps of mimmillan teeth. Never 

 before hat the tri'ul)crcular theory been so lucidly explained, so 

 logically followed out ; never before have its weaknesses been 

 10 obvious, its errors so plain. 



Prof. Oiiborn first calls attention lo "Cope's dcminslrationof 

 the Iritubercular molar as the central type in all (he mam- 

 malia" a» " a great step forward. In looking over the odonto- 

 graphies of Cuvier, Owen, Tome?, and liium:, we find there is 

 no suspicion of this common type around which th: highly 



I " The Rise of the Marntn %lta io N inh A-nifca." Stu Jic« from the IJioI. 

 L«bKatoricf of Co*umSia C>llpj;e, /.aai. vjl, i. iS^j ; repr,nlcd in Nature, 

 No*. 13^3 and 1363, vol. xlix. 1S91. 



diverse mammalian molars centre." Further on he states that 

 "all the specialised mammalian series, ungulates, primates, 

 carnivores, insectivores, rodents, and marsupials are found play- 

 ing similar, yet independent adaptive variations upon one type," 

 that of the Marsupials and Placentals " every kno«n triassic, 

 Jurassic, cretaceous, and basal eocene fossil (exccptini; Dicio- 

 cynodon) is in some stage of trituberculy," and that he is "able 

 to bring forward evidence that the multitubercular molar in- 

 stead of being primitive was derived from the tritubercular." In 

 short, the arguments from paK^ontology and embryology in 

 favour of considering the tritubercular pattern of molar as the 

 primitive type are forcibly put before us. The place occupied 

 by this common type amongst teeth, is compared lo that held by 

 the pentadaclyle type in the morphology of the limbs of the four- 

 footed vertebrates. What trituberculy is for the teeth, " pen- 

 tadactyly has long been for the feet, "and later, "themol.-iisof the 

 clawed and hoofed mammals can now be compared, as we com- 

 pare the hand or foot of the horse wi'h that of the cat, because 

 they spring from a common type." .\t the risk of being tedioiH, 

 I have thought it necessary to give all these quotations to make 

 Prof. Osborn's position quite clear. 



What is our astonishment, then, when a little further on we 

 come upon the statement that, " upon the polyphyletic theory 

 of the origin of the mammals here advocated [namely. 

 the independent origin of the Monotremes, Marsupials, an I 

 Placentals from a common ancestral stock, the Pro-mammalia |. 

 we must admit, first, the independent evolution of trituberculy 

 in diflerent phyla :and second, the branching off of several gre.T 

 groups in the pre-tritubercular stages." (!) We are then told that 

 the Prototheria, the Metatheria, the Insectivora, and the higher 

 Placentals have all independently, and more or less rapidly, 

 entered "into trituberculy." 



How do these statements agree with the evidence menlioncl 

 above? What becomes of the comparison with foot structure 

 Are we to believe that the pentadaclyle limb has been considercil 

 to be the common or central type, because the various vertebraie 

 groups have acquired it independently ? The words " common " 

 and "central," as applied to a type of structure, h.ave no 

 significance nowadays unless equivalent to ancestral. It seems 

 hardly necessary to point out that such mythical types hovering 

 over organs, and compelling ihem to assume a certain form, 

 have no place in modern biology. We might be willing to 

 accept the tritubercular as a generalised, archaic, or anceUral 

 type ; but it is out of the question, at the same time, to claim 

 that it has been independently acquired by the groups in which 

 it occurs. Could the divergence in general structure, and 

 habits within the Marsupials, and the Pl.icentals, lead to a con- 

 vergence to one type of tooth? And, more extraordinary still, 

 to the same type in both cases? 



Moreover, many of the mammals, which, according to Prof. 

 Osborn, so readily p.ass "into trituberculy," only do so, 

 apparently, to pass no less readily out of it. .\ccording to this 

 theory the living forms which possess triconodont teeth, amongst 

 the Marsupials on the one hand and the Placentals on the other, 

 have been derived from ancestors with triconodont molars, 

 which passed through the tritubercular, and again back into the 

 triconodont type. 



There is a very grave objection to such a fickle mode of cusp 

 development, which seems to have escaped the notice of the 

 supporters of the theory. All the various types of leclh met 

 with amongst the mammalia are adaptations to particular kinds 

 of food, and methods of feeding ; the appearance or disappear- 

 ance of a few cusps here and there may seem a matter of trivial 

 importance to anyone forming a theory of cusp development, 

 but there is no reason to think that it is so to the animal which 

 possesses the teeth. The independent pa,ssage of all these 

 groups of mammalia through a triconodont stage would imply, 

 that they all and severally took to a pailicular kind of food (that 

 (or which the triconodont molar is an adaptation), and again 

 their passage into a tritubercular stage would imply, that they 

 afterwards took to another method of feeding (that for which 

 the tritubercular molar is an .adaptation). It is evident that the 

 difficulties encountered in the attempt lo derive the tritubercular 

 tooth from a triconodon form in each group are overwhelming ; 

 we must, therefore, fall back on the supposition that the original 

 mammalian leeih were provided with many cusps, not placed in 

 one line, and the exact pattern of which remains still to be foun.l 



There is much evidence for the view (hat the upper mol.irs il 

 the Pro-mammalian ancestor were of the tritubercular, and tli 



NO. 1279. VOL. 50] 



