Dec. 21, 1876] 



NATURE 



16 



pretty completely cut oflf from one another, three animals, 

 in parentage widely dissimilar, have arrived at dentitions 

 oi rodent type. Thus in Austral a, a region practically 

 wholly monopolised by marsupials, a marsupial, the 

 Wombat, has a dentition very much like an ordinary pla- 

 cental rodent. In the island of Madagascar, which is, 

 with the exception of a few mice, without indigenous 

 rodents, a lemurine animal, the Cheiromys [Aye-aye], has 

 a dentition modified in a similar direction ; and else- 

 where, scattered over the world, we have the ordinary 

 rodents. In fact, three creatures, as widely different in 

 parentage as they well could be, have been modified by 

 natural selection until they have dentitions, not identical, 

 but for practical purposes not unlike." 



In one instance we think that Mr. Tomes has gone a 

 little too far in his generalising proclivities, and this is 

 with reference to the canine teeth, when we are told that "it 

 would practically be very inconvenient to abolish the term 

 canine, but it shouid be borne in mind that its signifi- 

 cance is merely equivalent to caninifor7n premolar^ and 

 that in describing the dog's dentition we should be less 

 liable to be misinterpreted were we to say that it has five 

 premolars, of whicti the first is caniniform." This un- 

 willingness to recognise the canine tooth as an element 

 of the dental series makes Mr. Tomes, as do M. Milne- 

 Edwards and some other naturalists, include the lower 

 " canines " of the ruminant ungulata, and lemurs, with the 

 incisors. This, however, is quite opposed to the well- 

 supported doctrine that in placental mammals there are 

 never more than three incisors in each side of each jaw, 

 and if extended to its logical consequences must render 

 it necessary that the lower incisors of all mammals should 

 be termed incisiform premolars, a very awkward predica- 

 ment. We all accept it as a fact that the definition of a 

 " canine " tooth is not established upon so distinct a 

 footing as a premolar and a molar, or an upper incisor ; 

 but any argument which attempts to annihilate its entity 

 does away with the lower incisors also. In an animal 

 like the Musk Deer {Moschus moschiferus), where the 

 premolars are gradually reduced from behind forwards, 

 how is it, it may be asked, that the upper canine tooth 

 does not, if a premolar, participate in the reduction ? Re- 

 versely it is immensely exaggerated in size. 



Attention is drawn, to an important fact recently arrived 

 at by M. Pietkewickz, that, contrary to the statement of 

 Goodsir, there are no traces, even in the youngest 

 examples, of rudimentary upper incisors in the true 

 Ruminantia. 



There is another minor point in which we would differ 

 from Mr. Tomes. Speaking" of the Perissodactylate and 

 Artiodactylate Ungulata, it is said that " the distinction 

 between the two groups is strongly marked, if living 

 animals alone be considered ; but, as Prof. Huxley has 

 pointed out, increasing knowledge of fossil forms is 

 tending to break down the line of demarcation." Our 

 experience is otherwise, and we cannot see between 

 Coryphodon and Anoplotheriwn any nearer affinities than 

 between the Tapir and the Hippopotamus. It is quite 

 beyond our comprehension that an animal with the axis 

 of the limb running through the middle of the median 

 digit should be allied to a similar creature in which the 

 axis runs between the third and fourth digits, except in 

 times when no such special axis of support existed ; 

 that is, before the Ungulata came into existence as 

 such. 



In conclusion we cannot do better than recommend 

 this valuable work by Mr. Tomes to students, not onjly to 

 those who make the diseases of the teeth their special 

 study, but also to others who are endeavouring to obtain 

 reliable information on the comparative anatomy of these 

 organs, which from their variations and complexity in 

 different animals, have yielded and for a long time yet to 

 come will continue to yield', so large a field for zoological 

 investigation. 



FORMA TION OF RAINDROPS AND HAIL- 

 STONES^ 



WHEN the particles of water or ice which constitute 

 a cloud or fog are all of the same size, and the air 

 in which they are sustained is at rest or is moving uni- 

 formly in one direction, then these particles can have no 

 motion reliatively to each other. The weight of the par- 

 ticles will cause them to descend through the air with 



FiG. I. — Perfect Hailstone. 



velocities which depend on their diameters, and since 

 they are all of the same size, they will all move with the 

 same velocity. 



Under these circumstances, therefore, the particles will 

 not traverse the spaces which separate them, and there 

 can be no aggregation so as to form raindrops or hail- 

 stones. 



If, however, from circumstances to be presently con- 

 sidered, some of the particles of the cloud or fog attain a 

 larger size than others, these will descend faster than the 

 other-s, and will consequently overtake those immediately 

 beneath them ; with these they may combine so as to form 

 still larger particles which will move with greater velo- 

 city, and more quickly overtaking the particles in front of 

 them will add to their size .at an increasing rate. 



Under such circumstances, t-herefore, the cloud would 



Fig. 2. — Broken Hailstone. 



be converted into rain or hail according as the particles 

 were water or ice. 



The size of the drops from such a cloud would depend 

 simply on the quantity of water suspended in the space 

 swept through by the drop in its descent, that is to say, 



' Abstract of paper "On the Manner in whick Raindrops and Hailstones 

 are Formed.'' by Prof. Osborne Reynolds, M.A , read at the Literary and 

 Philosophical Society, Manchester. 



