26 



NA TURE 



[May io, 1894 



environment as the strictly physical components ; and 

 that inasmuch as these coexisting organisms are discon- 

 tinuous species, the element of discontinuity may thus be 

 introduced. This is true, but it does not help in the 

 attempt to find the cause of the original discontinuity of 

 the coexisting organisms." 



Now since the deposition of the earliest paUcojoic rocks 

 animals have demonstrably been surrounded by such 

 "discontinuous'' organic environment. The statement, 

 that the environmental conditions " form a continuous i 

 series,'' is therefore untrue of all animals known to us. 

 If it has ever been true, we cannot know. The question, 

 whether the first living things which appeared upon the 

 earth were alike or not, is as unprofitable as speculation 

 about the beginning or the ending of any part of the 

 order of things from which our experience is derived must 

 always be. 



These preliminary arguments in favour of Mr. Bateson's 

 main contention therefore fail, when applied to any part 

 of the process of evolution of which wc can know any- 

 thing. It remains to consider what experimental evidence 

 is brought forward to prove that variation is in fact " dis- 

 continuous" in any living animals. 



No definition of what exactly is meant by "discon- 

 tinuous" variation is given ; and the conception adopted 

 is difficult to grasp, since such domestic animals as the 

 bull-dog are taken as examples of its occurrence. It will 

 therefore be necessary to examine the treatment of some 

 special group of variations : and for this purpose the 

 chapters on teeth may be selected. The question is pro- 

 pounded — 



" What is the least size in which a given tooth can be 

 present in a species which sometimes has it and some- 

 times is without it?" 



And it is remarked that — 



"Considered in the absence of evidence it might be 

 supposed that any tooth could be reduced to the smallest 

 limits which are histologically conceivable ; that a few 

 cells might take on the character of dental tissue. . . . 

 Indeed, on the hypothesis that variation is continuous, 

 this would be expected.'' 



Mr. Bateson considers his evidence sufficient to show 

 that " the least size of a tooth is different for different 

 teeth and for different animals," and that therefore varia- 

 tion in the teeth of those animals discussed by him pro- 

 ceeds by integral steps, the magnitude of the step 

 difTering in different cases. But he gives no evidence 

 that he has ever looked for teeth reduced to the smallest 

 histological limits. The greater number of observations 

 recorded arc made upon dry skulls, in which such rudi- 

 ments could not be demonstrated. It is notorious that in 

 many animals, such as marsupials and whales, a whole 

 set of teeth exists, which can only be demonstrated by 

 careful histological examination of the entire jaw, at a 

 definite period in the life of these animals. But more 

 important is the fact that teeth have been demonstrated, 

 consisting of a few cells, which have hardly progressed 

 beyond the histological condition of an enamel germ, 

 in several of the animals used by Mr. Bateson to 

 support his view. These cases {jCercopilluciis, probably 

 Cynocephatus, the dog, and the cat) have been completely 



NO. I ?8o, VOL. 50] 



overlooked by Mr. Bateson ; and similar cases in other 

 vertebrates have been recorded. The condition, which 

 is said to be necessary on the hypothesis that variation 

 is continuous, does, therefore, in fact occur ; and the 

 contention as to the least possible size of particular 

 teeth fails. 



In somewhat similar cases Mr. Bateson lays stress upon 

 the rarity of slight variations, as compared with more 

 considerable abnormalities. In the present case, few 

 persons have examined the jaws of mammals in such a 

 way as to enable them to find the smaller abnormalities 

 and there is no evidence worth discussion which shows 

 whether they are rare or not. The only case in which 

 large numbers of jaws have been examined by proper 

 methods is that of man, in whom a fourth molar tooth is 

 commonly present as a mere uncalcified rudiment, the 

 cases in which this extra tooth becomes calcified and 

 breaks through the jaw being rarer in proportion 

 to the perfection of the extra tooth. .V similar con- 

 tinuous series of variations is presented by the 

 " wisdom tooth,'' which is most commonly a fairly 

 perfect molar, cases of imperfection being rarer in 

 proportion as the tooth approximates to the condition 

 of " a few cells," hardly " taking on the character of 

 dental tissue" at all, which it does occasionally assume. 

 It is therefore curious, that in commencing his attempt 

 to exhibit the discontinuity of dental variation, 

 Mr. Bateson should dismiss the case of human leetli 

 with the remark — 



" I do not know that among these human variations 

 are included phenomena different in kind from those seen 

 in other groups, except perhaps certain cases of teeth 

 united together." 



Surely variation which proceeds by integral steps ol 

 the magnitude of a tooth may justly be held to differ in 

 kind from variation which proceeds by indefinitely small 

 gradations ': 



In the case of teeth, and in many other cases discussed, 

 the method employed is not adapted to a determination 

 of the least possible magnitude of variations. For m 

 judging evidence based largely upon museum piepar.i- 

 tions and on printed records, it must always be remem- 

 bered that there is a tendency among curators and others 

 to regard a slight abnormality as not worth bottle, spirit, 

 and a place on the shelf, or to think it too trivial for 

 printed record. Anyone who has tried to obt.iin speci- 

 mens for a museum knows that many persons will take 

 pains to present a rare or striking specimen, who cannot 

 be induced to send quite common things. 



The only way in which the question can be settled for 

 a given variation seems to be by taking large numbers of 



I anmials, in which the variation is known to occur, at 

 random, and making a careful examination and record 

 of each. Mr. Bateson's chapter on teeth, like all his 



i chapters, is of great interest, and will doubtless serve to 

 throw important light on mariy things. But a careful 

 histological account of the jaws of five hundred dogs 

 would have done more to show the least possible size of 

 a tooth in dogs than all the information so painfully col- 

 lected. 



And so in many other cases. 



W. F. 



R. \V lit DON. 



