May II, 1876] 



NATURE 



33 



the matter is not free from difficulty, for the difference of 

 intensity with which a lateral sound is perceived by the two 

 ears is not great. The experiment may easily be tried 

 roughlf by stopping one ear with the hand, and turning 

 round backwards and forwards while listening to a sound 

 held steadily. Calculation shows, moreover, that the 

 human head, considered as an obstacle to the waves of 

 sound, is scarcely big enough in relation to the wave- 

 length to give a sensible shadow. To throw light on this 

 subject I have calculated the mtensity of sound due to a 

 distant source at the various points on the surface of a 

 fixed spherical obstacle. The result depends on the ratio 

 (a) between the circumference of the sphere and the 

 length of the wave. If we call the point on the spherical 

 surface nearest to the source the anterior pole, and the 

 opposite point (where the shadow might be expected to 

 be most intense) the posterior pole, the results on three 

 suppositions as to the relative magnitudes of the sphere 

 and wave-length are given in the following table : — 



Intensity. 



( Anterior pole '690 



a = 7, "I Posterior pole "318 



(Equator "356 



( An'erior pole... 

 I < Posterior pole 

 ( Equator 



•503 

 •285 



•237 



! Anterior pole "294 

 Posterior pole "260 

 Equator "232 



When, for example, the circumference of the sphere is 

 but half the wave-length, the intensity at the posterior 

 pole is only about a tenth part less than at the anterior 

 pole, while the intensity is least of all in a lateral direc- 

 tion. When a is less than ^, the difference of the inten- 

 sities at the two poles is still less important, amounting 

 to about one per cent, when a = j. 



The value of a depends on the wave-length, which may 

 vary within pretty wide limits, and it might be expected 

 that the facility of distinguishing a lateral sound would 

 diminish when the sound is grave. Experiments were 

 accordingly tried with forks of a frequency of 128, but no 

 greater difficulty was experienced than with forks of a fre- 

 quency of 256, except such as might be attributed to the 

 inferior loudness of the formei". According to calculation 

 the difference of intensity would here be too small to 

 account for the power of discrimination. 



PROF. HUXLEY'S LECTURES ON THE EVI- 

 DENCE AS TO THE ORIGIN OF EXISTING 

 VERTEBRATE ANIMALS "■ 



VI. 



IN the highest group of Vertebrates, the Mammalia, the 

 perfection of animal structure is attained. It will 

 hardly be necessary, indeed it will be impossible, in the 

 time at our disposal, to give the general characters of 

 the group, but our purpose will be answered as well by 

 devoting a short time to considering the peculiarities of a 

 single well-known animal, the evidence as to the origin 

 of which approaches precision. 



The horse is one of the most specialised and peculiar 

 of animals, its whole structure being so modified as to 

 make it the most perfect living locomotive engine which 

 it is possible to imagine. The chief points in which its 

 structure is modified to bring about this specialisation, 

 and in which, therefore, it differs most markedly from 

 other mammals, we must now consider. 



In the skull the orbit is completely closed behind by 

 bone, a character found only in the most modified mam- 

 mals. The teeth have a very peculiar character. There 



' A course of six lectures to working men, <lelivered in the theatre of the 

 Royal School of Mines. Lscture VI., April 3. Continued from vol. xiii. 

 p. 516. 



are, first of all, in the front part of each jaw, six long 

 curved incisors or cutting teeth, which present a singular 

 dark mark on their biting surfaces, caused by the filling 

 in of a deep groove on the crown of each tooth, by the 

 substances on which the animal feeds. After the incisors, 

 comes on both sides of each jaw a considerable tooth- 

 less interval, or diastema, and then six large grinding 

 teeth, or molars and premolars. In the young horse a 

 small extra premolar is found to exist at the hinder end of 

 the diastema, so that there are, in reality, seven grinders 

 on each side above and below ; furthermore, the male 

 horse has a tusk-like tooth, or canine, in the front part of 

 the diastema immediately following the last incisor. Thus, 

 the horse has, on each side of each jaw, three incisors, one 

 canine, and seven grinders, making a total of forty-four 

 teeth. 



The grinding surfaces of the molars and premolars are 

 very curious. In the upper jaw, each tooth is marked by 

 four crescentic elevations, concave externally, the inner 

 pair having each a curious folded mass connected with it. 

 These projecting marks are formed of dentine and enamel, 

 and, consequently, wear away more slowly than the inter- 

 vening portions of the tooth, which are composed of 

 cement. The lower grinders are marked with two cres- 

 cents and two accessory masses, but the crescents are 

 convex externally, and, consequently, when the opposite 

 teeth bite together, the elevations do not correspond at 

 any point. In this way a very perfect grinding surface 

 is obtained. The teeth are of great length, and go on 

 growing for a long time, only forming roots in old animals. 

 AH these points contribute to the perfection of the horse 

 as a machine, by rendering the mastication of the food, 

 and its consequent preparation for digestion in the 

 stomach, as rapid and complete a process as possible. 



It is, however, in the limbs that the most striking devia- 

 tion from the typical mammalian structure is seen, the 

 most singular modifications having taken place to pro- 

 duce a set of long, jointed levers, combining great strength 

 with the utmost possible spring and lightness. 



The humerus is a comparatively short bone inclined 

 backwards : the radius is stout and strong, but the ulna 

 seems to be reduced to its upper end — the olecranon or 

 elbow ; as a matter of fact, however, its distal end is left, 

 fused to the radius, but the middle part has entirely dis- 

 appeared : the carpus or wrist — the so-called " knee " of 

 the horse — is followed by a long " cannon-bone," attached 

 to the sides of which are two small " splint-bones " ; the 

 three together evidently represent the metacarpus, and it 

 can be readily shown that the great cannon-bone is the 

 metacarpal of the third finger, the splint-bones those of 

 the second and fourth. The sphnt-bones taper away at 

 their lower ends and have no phalanges attached to them, 

 but the cannon-bone is followed by the usual three pha- 

 langes, the last of which, the "coffin-bone," is ensheathed 

 by the great nail or hoot. 



The femur, like the humerus, is a short bone, but is 

 directed forwards ; the tibia turns backwards, and has the 

 upper end of the rudimentary fibula attached to its outer 

 angle. The latter bone, like the ulna, has disappeared 

 altogether as to its middle portion, and its distal end is 

 firmly united to the tibia. The foot has the same struc- 

 ture as the corresponding part in the fore-limb — a great 

 cannon-bone, the third metatarsal; two splints, the second 

 and fourth ; and the three phalanges of the third digit, the 

 last of which bears a hoof. 



Thus, in both fore and hind limb one toe is selected, 

 becomes greatly modified and enlarged at the expense of 

 the others, and forms a great lever, which, in combination 

 with the levers constituted by the upper and middle divi- 

 sions of the limb, forms a sort of double C-spring arrange- 

 ment, and thus gives to the horse its wonderful galloping 

 power. 



In the river-beds of the Quaternary age — a time when 

 England formed part of the Continent of Europe — 



