August 1, 1890.] 



KNOWLEDGE. 



195 



more numerous than in the tooth represented in Fig. 15, 

 so that the even surfaces of dentine form still narrower 

 strips. At first sight it seems difficult to believe that the 

 Elephant now inhabiting the burning plains of India 

 should be closely allied to a species which formerly 

 roamed over the icy regions of Siberia, but there are two 

 considerations which show how little value such objections 

 have. Thus, in the first place, there is considerable evi- 

 dence that the climate of Siberia, although doubtless 

 always cold in winter, was formerly less severe than at 

 present. We have, moreover, evidence in the case of the 

 Tiger how an animal can support the extremes of heat and 

 cold with no alteration of its structure. Many people, 

 indeed, if they were asked to mention the Ii/iliitut of the 

 Tiger, would say India, little knowing that this creature 

 ranges in China and thence to Siberia into extremely high 

 latitudes. The skins of these northern tigers are far 

 handsomer than those from India, the hair being long and 

 comparatively shaggy, so as to protect its owner from the 

 bitter cold. The case of the Indian Elephant and the 

 Mammoth is a piecisely similai one, the eMstmg Indian 

 species having, as we all know m almost naked skin, 



while the Siberian Mammoth was clothed with long 

 shaggy hair, as we learn not only from its frozen remains, 

 but also from the rude pictures of the living animal drawn 

 on fragments of its own tusks by the old pre-historic 

 hunters of the Dordognc many centuries ago. 



In menlioiiing tusks, we should not forget that the 

 modern Elephants differ from many of the old Mastodons 

 in having tusks only in the upi)er jaw, while in the latter 

 they were also present in the lower. We have, therefore, 

 here also another instance of the greater specialisation of 

 the recent types. 



In showuig that with the advance of time the Elephants 

 have gradually developed an extremely complex type of 

 grinding tootii from a comparatively simple one, we find 

 that they occupy a parallel position with that held by the 

 other two great groups of the Ungulate order. 



There is, however, one very impcn'tant point whereby 

 the Elephants differ from these two groups, namely, that 

 they have not undergone any contemporaneous modifica- 

 tion ill tlie structure of the foot. The foot of an Elephant 

 is, indeed, of an exceedingly primitive type, having live 

 complete toes, and being more like those of the very 

 earliest Ungulates of the Tertiary period than is the case 

 with any other living member of the order, except the little 

 Hyrax or so-called Coney of the Bible. Tlie explana- 

 tion of this absence of modilicalion in the structure of the 

 foot of the recent as compared with the extinct Elephants 

 is, however, not far to seek. ]5y reason of their huge bulk 

 these animals have no need to fear the attacks of other 



creatures, so that there was no necessity for the develop- 

 ment of a limb suitable for extreme speed, while the huge 

 bulk of their bodies of itself also required the retention of 

 a type of limb constructed on principles of strength rather 

 than elegance. Hence there has been no iuducement for 

 any alteration in the structure of the feet of these pon- 

 derous brutes as time rolled on ; and so that while it requires 

 the aid of the man of science to trace the relationship 

 existing between the tooth of the ancient Mastodon and 

 that of the modern Indian Elephant, the veriest tyro 

 would not hesitate to declare that the skeleton of the same 

 two creatures indicated the closest relationship. 



To sum up the results of this brief survey of some of 

 the more strikmg features connected with the cheek-teeth 

 of the Ungulate Mammals, we may say that all the Ungu- 

 lates of the lower Tertiary deposits had low-crowned teeth 

 of comparatively simple structure ; and that those of all 

 the three groups into which the order is divided can 

 be derived from a type of tooth not far removed fi-om that 

 possessed by the ancestors of the Pigs, the latter type being 

 itself a modification from the still more primitive triangular 

 type bi all the thiee groups of Ungulates, the crowns of 

 the teeth have tended to in- 

 crease in height and in com- 

 plexity of structure with the 

 advance of time ; those I'ngu- 

 lates with the tallest crowned 

 teeth being characteristic of 

 the most recent period of the 

 earth's history. Finally, that 

 while in the groups contain- 

 ing the Ruminants and the 

 Horses the structure of the 

 feet has been modified imri 

 piissu with that of the teeth, 

 to produce hmlis capable of 

 carrying their owniers at an 

 extremely rapid rate, in the 

 Elephants, where there is no 

 need for swiftness of flight, the 

 feet have not undergone this adaptive modification, so 

 that the whole structural alterations have been confined to 

 the teeth. 



In a subsequent communication we shall hope to illus- 

 trate some of the modifications of tooth-structure found in 

 other groups of Mammals ; but we venture to think that 

 sufficient has been said to show what an extremely 

 interesting study is that of teeth. These organs are, in- 

 deed, interesting not only from their curious and often 

 beautiful forms, but also, as we have striven to show in the 

 foregoing paragniphs.as affording a concise and, so to speak, 

 epitomised sumiuary of the degree of evolution which any 

 particular group of animals has imdergone in the course of 

 time. 



WEISMANN'S THEORY OF HEREDITY. 



By Enw.\RD Clodd. 



HE scientific world is becoming interested to the 

 point of agitation about certain theories which 

 have been broached by a very distinguished 

 Cierman biologist, Professor August Weismann," 

 of Freiburg. These theories have been set forth 

 and defended in a series of essays, issued at intervals 



* Hfgai/ii upon Heredity mid Kindred Biological Problems. By Dr. 

 August Weismftim. Autlioiised Translntion, Edited by E. B. Poulton, 

 Solmar Schijnland, and Artlmr E. Shipley. (Oxford : Claroiidoii Press, 

 188y.) 



T 



