162 



KNOWLEDGE, 



[September 1, 1898. 



at will, and are only opened at such times as the creatures 

 come to the surface to breathe, when a column of water is 

 generally thrown up by the rush of expired air let loose 

 shortly before the head reaches the surface. In all theu- 

 internal structures, as well as in the mode of production 

 and nourishing of their young, cetaceans conform strictly 

 to the ordinary mammalian type ; and we accordingly see 

 that their assumption of a fish-like form is, with the 

 exception of the loss of the hind limbs and the modifi- 

 cation of the front pair into flippers, mainly superficial. 

 In departing from the fish-type in having the expansion 

 of the tail-tin horizontal instead of vertical, the necessity 

 of having an organ capable of bringing them rapidly 

 to the surface has been the inducing cause ; while in 

 order to prevent their blood from being reduced below 

 the proper temperature by the chill of the surrounding 

 water, the whole body is invested with a thick layer 

 of oily fat, commonly known as the blubber, beneath the 

 skin. 



As we have had occasion to mention incidentally in 

 previous articles, existing cetaceans are divisible into two 

 "reat groups, distinguished as the whalebone whales and 

 the toothed whales; the latter group including sperm- 

 whales, together with the grampuses, porpoises, and 

 dolphins. The most obvious distinction between these two 

 groups, as the terms applied to them indicate, relates to the 

 presence or absence in the adult condition of true teeth. 



Fia. 2.— Tlie last six upper teeth of the Killer Wliale. (After Sir 

 ^X. H. Flower.) 



Confining our attention in the present article to the 



toothed cetaceans, of which the dolphins and their allies 



are the best specialized representatives, we find that the 



two jaws may be in some cases provided with a full series 



of teeth, while m other forms the number of teeth may 



be reduced to a smgle pair, or even, as in the male 



nai'whal, to a solitary tusk. Whether, however, the teeth 



be many or few (and in the female narwhal there are 



none of any functional importance), the structure known 



as whalebone is never developed in the mouth ; wliile all 



the members of the group are further distinguished from 



the whalebone whales by the circumstance that the 



nostrils invariably open by a single external aperture, 



which is very frequently in the form of a transverse 



crescentic sUt, closed by an overhanging valve. In the 



latter respect these cetaceans are more specialized than 



are the whalebone whales ; and as the presence of teeth 



in the former indicates that they could not have been 



derived from the latter, it is evident that tlie two groups 



are of extreme antiquity, and have undergone a parallel 



development. Till recently, it has indeed been considered 



that they were divergent branches from some common 



ancestral type ; but, as mentioned in our article on 



" Parallelism in Development," it has been lately suggested 



that each may have had a totally distinct origin, although 



the evidence in favour of such a view is, at present at 



least, far from conclusive. 



In the structure of their teeth, the modern-toothed 

 whales differ very widely from the generality of mammals. 

 In the first place, their teeth are always of the simple 



structure shown in Fig. 2, having conical or compressed 

 crowns, and undivided roots ; while, secondly, there is 

 only one single series developed, the replacement of the 

 anterior ones characterizing the majority of mammals 

 being wanting. From this simple structure of their teeth 

 it has been argued that these cetaceans are among the most 

 primitive of all mammals ; but, altogether apart from the 

 conclusive evidence that all whales (as proved by their 

 breathing air) are derived from land mammals, it has 

 recently been shown by the researches of Dr. Kiikenthal, of 

 Jena, that this view is quite untenable. By examining em- 

 bryos of young cetaceans of this group, that observer has 

 demonstrated that there actually are rudiments of a second 

 series of teeth, which, although never coming to maturity, 

 serve to show that there were once two complete sets, and 

 that the permanent teeth correspond, in part at least, to 

 the milk-teeth of other mammals ; thus indicating that the 

 present state of the cetacean dentition is a degraded one. 

 Hitherto it has not, indeed, been shown by embryology 

 that the teeth of this group were originally of a complex 

 type (although in the case of the whalebone whales this 

 has been demonstrated), but, fortunately, here palaeontology 

 comes to our aid. Thus in the middle of the tertiary 

 period there occur remains of what may be termed shark- 

 toothed dolphins (squalodonts), in which the permanent 

 teeth are differentiated into distinct series, corresponding 

 to the incisors, canines, pre-molars, and molars of other 

 mammals ; while, for all we know to the contrary, there 

 may also have been a regular replacement of the more 

 anteriorly placed teeth. In these shark-toothed dolphins 

 the molar teeth, instead of being of the simple structure 

 of those represented in our illustration, were severally 

 implanted in the jaws by two perfectly distinct roots ; 

 while their large, laterally comisressed, and somewhat fan- 

 shaped crowns were furnished with a number of cusps on 

 their hinder cutting-edges. Indeed, these teeth much 

 resemble the pre-molar tooth of a dog or the molars of a 

 seal ; and they obviously serve to indicate a transition from 

 the modern toothed cetaceans towards ordinary mammals. 

 This, however, is by no means all, since in a still earlier 

 portion of the same division of the tertiary there occur 

 other cetacean-like animals known as zeuglodonts, which 

 have still more complicated teeth, and otherwise depart 

 further from the modern cetacean type— so much so, 

 indeed, that they have been regarded by some writers 

 as more nearly allied to the seals. In our opinion they are, 

 however, undoubtedly primitive cetaceans, and thus serve, 

 not only to connect the present group with other mammals, 

 but also, in conjunction with the shark-toothed dolphins, to 

 show that the simple teeth of the former are clearly pro- 

 duced by degeneration from a complex type. As regards 

 the particular group of land mammals from which whales 

 were derived, we have stated in the article on " Swimming 

 Animals" that it has been thought that their nearest allies 

 are with the ancestors of the pig-hke hoofed mammals. 

 This, indeed, is the view of Sir W. H. Flower ; but we 

 confess that from the nature of the teeth of the two extinct 

 groups above mentioned, coupled with certain resemblances 

 of the skeleton of the zeuglodonts to those of the seals, we 

 are rather more inclined to look among flesh-eating land 

 mammals for the lost ancestors. Still, however, it must 

 be remembered that in the early eocene period, which is 

 probably the very latest epoch at which cetaceans could 

 have originated, the distinction between carnivorous and 

 hoofed mammals was but imperfect, so that, after all, the 

 ancestral cetacean stock may well have been of an 

 extremely generalized type. At present, however, we are 

 almost completely in the dark in all that concerns this 

 interesting subject. 



