450 



NA TURE 



[April 6, 1876 



and the baleen whales or Mysticetes, and none of the 

 known extinct forms present true transitional or interme- 

 diate characters ; but it must be remembered that even 

 such remains as have been already collected have not 

 yet been thoroughly worked out. The Mysticetes appear, 

 at first sight, the most specialised and aberrant, in the 

 absence of teeth, in the presence of whalebone or baleen, 

 in the form and size of the mouth ; but as we see in other 

 groups, dental characters and all such as relate to the 

 prehension of food generally, are essentially adaptive, 

 and consequently plastic or prone to variation, and hence 

 cannot well be relied upon as tests of affinity. In 

 another character, also adaptive, the laxity of the con- 

 nection of the ribs with the vertebral column and with 

 the sternum, and the reduction of that bone in size, 

 allowing great freedom of expansion of the thoracic 

 cavity, for prolonged immersion beneath the water, the 

 Mysticetes have passed beyond the Odontocetes in spe- 

 cialisation. On the other hand, the great symmetry 

 of the skull, the more anterior position of the nares, 

 and their double external orifice, the form of the nasal 

 bones, the presence of a distinctly developed olfactory 

 organ, the mode of attachment of the periotic bone to the 

 cranium, the presence of a CKCum, and the regular 

 arrangement of the alimentary canal, the more noi-mal 

 characters of the manus and the better development of 

 the muscles attached to it, and the presence (in many 

 species at least) of parts representing a hind limb, all show 

 less deviation from the general mammalian type than is 

 presented by the Odontocetes. Taking all their struc- 

 tural characters into consideration, as well as what we 

 know of their past history, it does not appear that we can 

 consider either type to have been derived from the other, 

 at all e\cnts in the form in which we see it now, but must 

 rather view them as parallel groups. 



Among the Mysticetes, in the especially distinguishing 

 characters of the division, the Balcenopto-a; are less 

 specialised than the Balance, which in the greater size of 

 the head, the length and compression of the rostrum, the 

 development of the baleen, and the shortness of the cer- 

 vical region, are exaggerated types of the former, and yet 

 they retain more fully some primitive characters, as the 

 better development of the hind limb, the pentadactylous 

 manus, and the absence of a dorsal fin. Both forms are 

 found distinct in a fossil slate as far back as the early 

 Pliocene, but generally represented by smaller species 

 than those now existing. The Mysticetes of the Miocene 

 seas were, as far as we know at present, only Balcejioptem, 

 some of which {Cetotherium, Brandt) were, in the elongated 

 flattened form of the nasal bones, the greater distance 

 between tiie occipital and frontal bones at the top of the 

 head, and the greater length of the cervical vertebrae 

 more generalised than those now existing. In the form 

 of the mandible they are considered by Van Beneden to 

 present more approximation to the Cetodonts. 



Among the toothed whales, the earliest known form, 

 the Zeiiglodot, was far the most aberrant, approach- 

 ing in the structure of its skull and teeth to a more gene- 

 ralised but very low carnivorous type. In smallness of 

 cerebral cavity, compared to the mouth and other parts 

 of the skull, it is as far below all other known cetaceans, 

 as the singular Arctoryon primcevus is below all carni- 

 vores. One could quite imagine that the skull of a very 

 degraded seal would present many features in common 

 with Zeuglodon, and this is the only near link we seem to 

 possess between the Cetacea and the rest of the animal 

 world. All the resemblances which some naturalists have 

 seen between the skull of Zeuglodon and the Sirenia are 

 purely superficial and imaginary. The forward position of 

 the narial aperture and the length and flatness of the 

 nasal bones which this animal possesses in common with 

 (though to a greater extent than) the Mysticetes, we may 

 suppose to be common primitive cetacean characters, 

 though completely lost in all other known Odontocetes. 



\ 



Even the Squalodons, which in dental characters so much 

 resemble the Zeuglodons as to have been placed in the 

 same genus by some zoologists, agree in their essential 

 cranial characters with the ordinary dolphins. They are, 

 in fact, dolphins with double-rooted molar teeth, peculiar 

 to the Miocene formations of Europe and America. 

 Among existing dolphins, Platanisia has been considered 

 to conform most to the general type of mammalian struc- 

 ture. It is therefore interesting to find a similar form 

 {Champsodelphis) well represented among the earliest 

 fossil remains of Cetaceans in Europe, and others abun- 

 dant in North America. Apart from these the greater 

 number of toothed whales range themselves under the 

 two principal heads of Ziphioids or Physeteroids, and 

 Dclphitioids. The former are an ancient group, of which 

 the remains are exceedingly numerous in the Antwerp 

 and Norwich crags, and of which the existing sperm 

 whale is a highly modified and specialised torm. Among 

 the latter, Delphinus and its various modifications may be 

 regarded as the dominating type of Cetaceans at the pre- 

 sent day, abundant in slightly differentiated species, and 

 abundant in individuals. They are in this respect to the 

 rest of the order, much as the hollow-homed ruminants 

 are to the Ungulates. 



It seems in vain at present to speculate upon the origin 

 of the Cetacea. They present no marks of closer affinity 

 to the lower classes of Vertebrates than do the rest of 

 their own class. Indeed, in all that characteristically 

 distinguishes a Mammal from the oviparous Vertebrates, 

 especially in the nervous and reproductive systems, they 

 are far above many other groups of the class. There is 

 no existing order of land mammals to which they can be 

 said to. be decidedly and unquestionably allied. Their 

 agreement with the Stroiia is mainly in modifications of 

 structure adapted for a somewhat similar mode of exist- 

 ence, while in many essentials the difference between 

 them is as wide as that between any other two orders. 

 Taking into consideration many of their habits, and their 

 food, and bearing Zctiglodon in mind, a relationship to 

 the Carnivora through the seals seems indicated ; but if 

 the mode of development has the weight many modern 

 zoologists are disposed to assign to it, their affinities 

 would be rather with the Ungulates, an order from which, 

 on other grounds, they are far removed. 



(To be continued.') 



ON REPULSION RESULTING FROM RADI. 



ATION—Pa^t IV. ^ 



T N this paper the author describes experiments on the 

 ■*■ repulsion produced by the different rays of the solar 

 spectrum. The apparatus employed is a horizontal beam 

 suspended by a glass fibre, and having square pieces 

 of pith at each end coated with lampblack. The whole is 

 fitted up and hermetically sealed in glass, and connected 

 with an improved mercury-pump. In front of the square 

 of pith at one end a quartz window is cemented to the 

 apparatus ; and the movements of the beam, when radi- 

 ation falls on the pith, are observed by a reflected ray of 

 light on a millimetre-scale. The apparatus was fitted up 

 in a room specially devoted to it, and was protected on 

 all sides, except where the rays of light had to pass, with 

 cotton-wool and large bottles of water. A heliostat 

 reflected in a constant direction a beam of sunlight, 

 which was received on an appropriate arrangement of 

 slit, lenses, and prisms for projecting a pure spectrum. 

 Results were obtained in the months of July, August, and 

 September ; and they are given in the paper graphically 

 as a curve, the maximum being in the ultra-red, and the 

 minimum in the ultra-violet. Taking the maximum at 



' Abstract of a paper read befor the Royal Society, Feb. 10, 1876, by- 

 William Cr»okes, F.R.S., &c. ,. . 



