I 



!6o 



NA rURE 



[January i i, 1894 



suggests developed aquatic habits ; as above noted, some of 

 this family acquired large brains, and persisted late into the 

 Miocene. A still more remarkable likeness to the cats is 

 exhibited in the Pal^onictis family, which, unlike the Hyte- 

 nodons, forms its sectorials out of exactly the same teeth as 

 the true cats. The first American Palaeonictis was found two 

 years ago by Wortman, and this author and myself have 

 suggested that this may be the long-sought ancestor of the 

 Felidre. The Civets are anticipated in the Proviverridre ; yet 

 both Cope and Scott, the highest authorities on this subject, 

 believe that the dog-like Miacidse alone formed the connecting 

 link between the Creodonta and the true Carnivora. 



The foot structure of the ancient Puerco ungulates is still 

 only partly known. Cope has divided these animals into the 

 Amblypoda and Condylarthra. The Amblypoda are repre- 

 sented in the Puerco by a large form called Pantolambda, with 

 selenodont triangular upper molars, and possibly by Perip- 

 tychus, with bunodont triangular molars. The Pantolambda 

 molais were, as Cope has shown, converted into those of Cory- 

 phodon, the great lophodont Amblypod of the Wahsatch, by a 

 process exactly analogous to that in which the anterior half of 

 a Pal^otherium molar was formed, that is, they acquired 

 outer and anterior crests but no posterior crests. This 

 Coryphodon molar type was still later converted into the Uin- 

 tatherium type by swinging around the outer crest into a 

 transverse crest. I have recently made a careful study of the 

 fore and hind feet of Coryphodon, and have found that while 

 the fore-foot was subdigitigrade like that of the elephant, the 

 hind-foot was fully plantigrade, the entire sole resting upon 

 the ground. The relation or connection between the Bridger 

 Dinocerata and these earlier Amblypoda is still unknown. The 

 Puerco Periptychus left no descendants. The other ungulates 

 of the Puerco were the Condylarthra, the primitive Phena- 

 codontidse, the supposed ancestors of the Arliodaclyls and 

 Perissodaotyls. Much remains to be done to clear up this 

 question. 



Thus an immense number of problems still await solution, 

 and demand the generous co-operation of European and Ameri- 

 can specialists in the use of similar methods of research, in the 

 prompt publication of descriptions and figures, and in the free 

 use of museum collections. I may be pardoned for calling 

 general attention to the service which the palceontological de- 

 partment of the American Museum is trying to render in the 

 immediate publication of stratigraphical and descriptive tables 

 of western horizons and localities. 



The Factors of Evolution. 



A few words in conclusion upon the impressions which a study 

 of the rise of the mammalia gives as to the factors of organic 

 evolution. I refer also to recent papers by Cope, Scott, and 

 myself. 



The evolution of a family like the Titanotheres presents an 

 uninterrupted march in one direction. While apparently 

 prosperous and attaining a great size, it was really passing into 

 a great corral of inadaptation to the grasses which were intro- 

 duced in the Middle Miocene. So with other families and 

 lesser lines extinction came in at the end of a term of develop- 

 ment and high specialisation. With other families no causes 

 for extinction can be assigned, as in the lopping off of the 

 smaller Miocene perissodactyls. The point is that a certain 

 trend of development is taken leading to an adaptive or in- 

 adaptative final issue — but extinction or survival of the fittest 

 seems to exert little influence en route. 



The changes en rozite lead us to believe either in predestina- 

 tion — a kind of internal perfecting tendency — or in kineto- 

 genesis. For the trend of evolution is not the happy resultant 

 of many trials, but is heralded in structures of the same form 

 all the world over and in age after age, by similar minute 

 changes advancing irresistibly from inutility to utility. It is an 

 absolutely definite and lawful progression. The infinite number 

 of contemporary developing, degenerating, and stationary cha- 

 racters preclude the possibility of fortuity. There is some law 

 introducing and regulating each of these variations, as in the 

 variations of individual growth. 



The limits of variation seem to lie partly in what I have 

 called the "potential of evolution." As the oosperm or fer- 

 tilised ovum is the potential adult, so the Eocene molar is the 

 potential Miocene molar. We have seen that the variations of 

 the horse and rhinoceros molars, apparently so diverse, are 

 really uniform — is not this evidence that the perissodactyl stem 



had these variations ?«/^/^«/?(r, waiting to be called foith by 

 certain stimuli? This capacity of similar development under 

 certain slimuli is part of the law of mammalian evolution, but 

 this does not decide the ciucial point whether the reaction is 

 spontaneous in the geim or inherited from the parent. I incline 

 to the latter opinion. H. F. Osborn. 



A DYNAMICAL THEORY OF THE ELECTRIC 

 AND LUMINIFEROUS MEDIUM.^ 



TT is only at the end of the last century that the somewhat 

 vague principle of the economy of action or effort in physical 

 actions — which, like all other general principles in the scien- 

 tific explanation of nature, is ultimately traceable to a kind 

 of metaphysical origin — has culminated in the hands of Lagrange 

 in his magnificent mathematical generalisation of the dynamical 

 laws of material systems. Before the date of this concise and 

 all-embracing formulation of the laws of dynamics there was 

 not available any engine of sufficient power and generality to 

 allow of a thorough and exact exploration of the properties of 

 an ultimate medium, of which the mechanism and mode of 

 action are almost wholly concealed from view. The precise 

 force of Lagrange's method, in its physical application, consists 

 in its allowing us to ignore or leave out of account altogether 

 the details of the mechanism, whatever it is, that is in operation 

 in the phenomena under discussion ; it makes everything depend 

 on a single analytical function representing the distribution of 

 energy in the medium in terms of suitable co-ordinates of 

 position and of their velocities ; from the location of this energy. 

 Its subsequent play and the dynamical phenomena involved in 

 it are all deducible by straightforward mathematical analysis. 



The problem of the correlation of the physical forces is thus 

 divisible into two parts, (i.) the determination of the analytical 

 function which represents the distribution of energy in the 

 primordial medium. which is assumed to be the ultimate seat of 

 all phenomena ; and (ii.) the discussion of what properties may 

 be most conveniently and simply assigned to this medium, in 

 order to describe the play of energy in it most vividly, in terms 

 of the stock of notions which we have derived from the ob- 

 servation of that part of the interaction of natural forces which 

 presents itself directly to our senses, and is form.ulated under 

 the name of natural law. It may be held that the first part 

 really involves in itself the solution of the whole problem ; that 

 the second part is rather of the nature of illustration and ex- 

 planation, by comparison of the intangible primordial medium 

 with other dynamical systems of which we can directly observe 

 the phenomena. 



The chief representative of exact physical speculation of the 

 second of these types has been Lord Kelvin. In the older 

 attempts of this kind the dynamical basis of theories of the 

 constitution of the aether consisted usually in a play of 

 forces, acting at a distance, between ultimate elements or 

 molecules of the medium ; from this we must, however, except 

 the speculations of Greek philosophy and the continuous 

 vortical theories of the school of Descartes, which were of 

 necessity purely descriptive and imaginative, not built in a 

 connected manner on any rational foundation. It has been in 

 particular the aim of I^ord Kelvin to deduce material pheno- 

 mena from the play of inertia involved in the motion of a 

 structureless primordial fluid : if this were achieved, it would 

 reduce the duality, rather the many-sidedness, of physical 

 phenomena to a simple unity of scheme ; it would be the ulti- 

 mate conceivable simplification. The celebrated vortex theory 

 of matter makes the indestructible material atoms consist in 

 \ vortex rings in a primordial fluid medium, structureless, homo- 

 \ geneous, and frictionless, and makes the forces between the 

 atoms which form the groundwork of less fundamental theories 

 consist in the actions excited by these vortices on one another 

 : through the inertia of the fluid which is their basis — actions 

 j which are instantaneously transmitted if the fluid is supposed 

 to be absolutely incompressible. 



In ca^e this foundation proves insufficient, there is another 



idea of Lord Kelvin's by which it may be supplemented. The 



characteristic properties of radiation, which forms so prominent 



! an element in actual phenomena, can be explained by the 



1 1 A paper read before the Royal Society on December 7, 1893, by Dr. 

 I Joseph Larmor, F.R.S., Fellow of St. John's College, Cambridge. 



NO. 1263, VOL. 49] 



