228 



NATURE 



\yan. 9, 1890 



of a number of entirely new elements — the rise of useful 

 structures from their minute embryonic, apparently useless, 

 condition, the most vulnerable point in the pure selection 

 theory. Here are opportunities we have never enjoyed 

 before in the study of the variation problem. 



The first undoubted ancestor of the horse is Hyraco- 

 therium; let us look back into the early history of its multi- 

 cuspid upper molars, every step of which is now known. 

 Upon the probability that mammalian teeth were developed 

 from the reptilian type, Cope predicted in 1871 that the 

 first accessory cusps would be found on the anterior and 

 posterior slopes of a single cone, i.e. at the points of 

 interference of an isognathous series in closing the jaws. 

 Much later I showed that precisely this condition is filled 

 in the unique molars of the Upper Triassic Dromotheriicm. 

 These with the main cusp form the three elements of the 

 tritubercular crown. Passing by several well-known 

 stages, we reach one in which the heel of the lower molars 

 intersects, and, by wearing, produces depressions in the 

 transverse ridges of the upper molars. At these points 

 are developed the intermediate tubercles which play so 

 important a role in the history of the Ungulate molars. 

 So, without a doubt, every one of the five main component 

 cusps superadded to the original cones, is first prophesied 

 by a point of extreme wear, replaced by a minute tubercle, 

 and grows into a cusp. The most worn teeth, i.e. the first 

 true molars, are those in which these processes take place 

 most rapidly. We compare hundreds of specimens of 

 related species ; everywhere we find the same variations 

 at the same stages, differing only in size, never in position. 

 We extend the comparison to a widely separate phylum, 

 and find the same pattern in a similar process of evolution. 

 Excepting in two or three side lines the teeth of all the 

 Mammalia have passed through closely parallel early 

 stages of evolution, enabling us to formulate a law : The 

 new main elements of the crown make their appearance at 

 the first points of contact and chief points of wear of the 

 teeth in preceding periods. Whatever may be true of 

 spontaneous variations in other parts of the organism, these 

 new cusps arise in the perfectly definite lines of growth. 

 Now, upon the hypothesis that the modifications induced 

 in the organism by use and disuse have no directive influence 

 upon variations, all these instances of sequence must be 

 considered coincidences. If there is no causal relation- 

 ship, what other meaning can this sequence have ? Even if 

 useful new adjustments of elements already existing may 

 arise independently of use, why should the origin of new 

 elements conform to this law ? Granting the possibility 

 that the struggle for existence is so intense that a minute 

 new cusp will be selected if it happens to arise at the 

 right point, where are the non-selected new elements, the 

 experimental failures of Nature ? We do not find them. 

 Palaeontology has, indeed, nothing to say upon individual 

 selection, but chapters upon unsuccessful species and 

 genera. Here is a practical confirmation of many of the 

 most forcible theoretical objections which have been 

 urged against the selection theory. 



Now, after observing these principles operating in the 

 teeth, look at the question enlarged by the evolution of 

 parallel species of the horse series in America and Europe, 

 and add to the development of the teeth what is observed 

 in progress in the feet. Here is the problem of correlation 

 in a stronger form even than that presented by Spencer 

 and Romanes. To vary the mode of statement, what 

 must be assumed in the strict application of the selection 

 theory ? (rt) that variations in the lower molars correlated 

 with coincident variations of reversed patterns in the 

 upper molars, these with metamorphoses in the premolars 

 and pocketing of the incisor enamel ; {b) all new elements 

 and forms at first so minute as to be barely visible 

 immediately selected and accumulated ; [c) in the same 

 individuals favourable variations in the proportions of the 

 digits involving readjustments in the entire limbs and 



skeleton, all coincident with those in the teeth ; {d) finally, 

 all the above new variations, correlations and readjust- 

 ments, not found in the hereditary germ-plasm of one 

 period, but arising fortuitously by the union of different 

 strains, observed to occur simultaneously and to be 

 selected at the same rate in the species of the Rocky 

 Mountains, the Thames Valley, and Switzerland ! These 

 assumptions, if anything, are understated. Any one of 

 them seems to introduce the element of the inconstant, 

 whereas in the marvellous parallelism, even to minute teeth 

 markings and osteological characters, in all the widely dis- 

 tributed forms between Hyracotherium and Equus, the 

 most striking feature is the constant. Viewed as a 

 whole, this evolution is one of uniform and uninterrupted 

 progression, taking place simultaneously in all the details 

 of structure over great areas. So nearly does race adapt- 

 ation seem to conform to the laws of progressive adaptation 

 in the individual, that, endowing the teeth with the power 

 of immediate reactive growth like that of the skeleton, we 

 can conceive the transformation of a single individual 

 from the Eocene five-toed bunodont into the modern 

 horse. 



The special application of the Lamarckian theory to 

 the evolution of the teeth is not without its difficulties, 

 some of which have been pointed out to me by Mr. E. B. 

 Poulton. To the objection that the teeth are formed 

 before piercing the gum, and the wear produces a loss of 

 tissue, it may be replied that it is not the growth, but 

 the reaction which produces it, which is supposed to be 

 transmitted. Again, this is said to prove too much ; why 

 is the growth of these cusps not continuous ? This may 

 be met in several ways : first, in the organism itself 

 these reactions are least in the best adapted structures, a 

 proposition which is more readily demonstrated in the 

 feet than in the teeth — moreover, since the resulting 

 growth never exceeds the uses of the individual, there is 

 a natural limit to its transmission ; secondly, the growth 

 of the molars is limited by the nutritive supply — we 

 observe one tooth or part growing at the expense of 

 another ; third, in some phyla we do observe growth 

 which appears to lead to inadaptation and is followed by 

 extinction. In one instance we observe the recession of 

 one cusp taking place pari passu with the development of 

 the one opposed to it. These and many more general 

 objections may be removed later, but they are of such 

 force that, even granting our own premises, we cannot 

 now claim to offer a perfectly satisfactory explanation of 

 all the facts. 



The evidence in this field for, is still much stronger than 

 that against, this theory. To sum up, the new variations 

 in the skeleton and teeth of the fossil series are observed 

 to have a definite direction ; in seeking an explanation of 

 this direction, we observe that it universally conforms to 

 the reactions produced in the individual by the laws of 

 growth ; we infer that these reactions are transmitted. If 

 the individual is the mere pendent of a chain (Galton), 

 or upshoot from the continuous root of ancestral plasm 

 (Weismann), we are left at present with no explanation 

 of this well-observed definite direction. But how can 

 this transmission take place? If, from the evident 

 necessity of a working theory of heredity, the ottus 

 probandiiaWs upon the Lamarckian — if it be demonstrated 

 that this transmission does not take place — then we are 

 driven to the necessity of postulating some as yet un- 

 known factor in evolution to explain these purposive or 

 directive laws in variation, for, in this field at least, the 

 old view of the random introduction and selection of new 

 characters must be abandoned, not only upon theoretical 

 grounds, but upon actual observation. 



Reading between the lines of Weismann's deeply 

 interesting essays, it is evident that he himself is coming 

 to this conclusion. Henry Fairfield Osborn. 



Princeton College, August 23. 



