April, 1903."] 



KNOWLEDGE. 



mentary condition of their known remains. Possibly 

 this may be accounted for on the supposition that the 

 evolution of primitive and presumably egg-laying 

 mammals from anomodout reptiles did not take place in 

 Europe, or indeed anywhere in the northern hemisphere ; 

 SI I that the few remains with which we are acquainted 

 were probably derived from individuals which had strayed 

 far away from the headquarters of the group. Nothing is 

 indeed more likelv than that Africa was the region where 

 the anomodouts develojjed into egg-laying mammals ; and 

 it is to that continent more than to any other part of the 

 world that we may look with the most hope to the 

 discovery of a full series of the intermediate forms between 

 reptiles and mammals. 



There is, indeed, a very great and very serious " im- 

 perfection of the geological record" in regard to the origin 

 of mammals. For after leaving the rich auomodont 

 fauna of South Africa, nothing is known of mammalian 

 development and history save such imperfect information 

 as can be gleaned from the aforesaid jaws from the 

 Stonesfield Slate, and others from the Pnrbeck deposits of 

 Dorsetshire, and the Triassic, Jurassic, and Cretaceous 

 r'ocks of the United States, till the early part of the 

 Tertiary peinod is reached. Then we come suddenly upon 

 an abundant and well differentiated mammalian fauna 

 widely spread over the northern hemisphere. The immediate 

 ancestors of this fauna we have yet to find. 



Nevertheless, in spite of this very large gap in our 

 knowledge, the information we already possess is, as 

 mentioned above, amply sufficient to demonstrate the close 

 affinity existing between mammals and the auomodont 

 reptiles, and to justify the assertion that (if evolution be 

 the true explanation of animal resemblances) the one group 

 is the descendant of the other. The thread of mammalian 

 development will be taken up again in a later paragraph ; 

 but before this is done, a few words must be devoted to 

 the other main groups of mammals. 



In the passage from Dr. Gadow's paper quoted above 

 it is incidentally mentioned that as reptiles have given 

 origin to mammals, so they themselves are the offspring 

 of am])hibians, or salamanders. And here, somewhat 

 curiously, we must once again refer to anomodonts. 

 Among the more mammal-like types in this group the 

 skull is vaulted, with the temporal region open, and the 

 component bones smooth, while the limbs were elongated 

 and adapted to raise the under surface of the bidy well 

 above the ground. On the other hand, there are certain 

 members of the group in which salamander-like features 

 predominate, the skull being depressed and expanded, with 

 the temporal region roofed over, and the cranial bones 

 sculptured externally, the limbs being short, so that the 

 under side of the body touched the ground. 



We thus have an approximation between the anomodonts 

 and the primeval salamanders, or labyrinthodonts. although 

 the exact nature of the relationship is at pi'eseiit somewhat 

 obscure. The labyrinthodonts, or, as they are often called, 

 stegoceplialians, display, however, an almost complete 

 passage into reptiles ; so much so, indeed, that certain 

 American Permian forms (i^ryo/w and Cricotus) are placed 

 by some writers in the first group and by others in the 

 S'icond. Among the most salamauder-like of true reptiles 

 are certain small forms (such as Hi/lonomus) from the 

 Carboniferous and Permian ; these in their turn pass into 

 still more advanced types, such as Frotorogaurug of the 

 upjx^r Permian, and PaJxohatteria of the Trias, and from 

 these latt(T there is an almost unbroken transition to the 

 New Zealand tuatera lizard {Sphciindon), by far the most 

 primitive and generalised oi all living reptiles. 



Regarding the connection between salamaudei'-like 

 am|ihibiiins — more especially the labyrinthodonts, or 



stegoceplialians — and fishes on the one hand and reptiles 

 on the other, we may once more, with slight verbal altera- 

 tion, quote from Dr. Gadow,* who writes as follows: — 

 " There is no doubt that the Amphibia have sprung from 

 fish-like ancestors, and that they in turn have given origin 

 to the Reptilia. The Amphibia consequently hold a very 

 important intermediate position. It was, perhaps, not a 

 very fortunate innovation when Huxley brigaded them 

 with the fishes as Ichthyopsida, thereby separating 

 them more from the Sauropsida (reptiles and birds) than 

 is justifiable- perhaps more than he himself intended. 

 The connecting link, in any case, is formed by the Stego- 

 c"phalia ; all the recent orders, the burrowing cnecilians, 

 salamanders and frogs, are far too specialised to have any 

 claims to the direct ancestral connections. The line 

 leading from the Stegocepbalia to fossil reptiles, notably 

 to such Proreptilia as Eryops and . Cricotus, and even to 

 the Prosauria (Hylonomus, Paloeohatteria, etc.), is 

 extremely gradual, and the steps are almost imperceptible. 

 Naturally, assuming evolution to be true, there must have 

 lived countless creatures which were neither Amphibia 

 nor Reptilia in the present intensified sense of the 

 systematist. The same consideration applies equally to 

 the line which leads downwards to the fishes. But the 

 great gulf within the Vertebrata lies between fishes and 

 Amphibia, between absolutely aquatic creatures with 

 internal gills and ' fins,' and terrestrial quadrupedal 

 creatures with lungs and fingers and toes." 



Since, as indicated in the closing sentence of this 

 passage, direct palseontological evidence of the descent of 

 salamanders from fishes is at present wanting, we shall 

 say no more with regard to those groups, except that the 

 indirect evide»ce, as Dr. Gadow points out, is of itself 

 sufficient to indicate the former existence of intermediate 

 types. We accordingly pass on to the fourth great group 

 of vertebrates, the birds. 



That modern birds come closer to reptiles than to any 

 other vertebrates is indicated by Huxley's scheme of 

 brigading the two groups together under the common title 

 of Sauropsida, to which allusion has been already made in 

 this article. To describe their common features in detail 

 would be quite out of place on the present occasion, but 

 it may be pointed out that both groups agree in having a 

 single condyle on the skull for articulation with the first 

 segment of the backbone, as well as in the ankle-joint 

 being situated in the middle of the tarsus instead of at the 

 upper end, and also by the frequent presence of what are 

 called uncinate processes to the ribs, and of a ring of bony 

 plates in the white of the eye. A certain approximation 

 to the reptilian type is, moreover, made by the lizard- 

 tailed birds (Arclueopteryx") of the .lurassie strata in the 

 retention of the long tail from which they derive their 

 popular title, as well as by the ]»reseuce of claws on two 

 of the digits of the wing, and of teeth in the jaws ; teeth 

 being also retained in several, if not all, of the birds of 

 the succeeding Cretaceous epocji. Nevertheless, Arclueop- 

 teryx is in allrespects essentially a bird, and in no sense 

 ail intenuediat*' form between birds and reptiles. 



There are, however, certain extinct reptiles, the Dino- 

 sauria, many of whose members habitually assumed an 

 upright posture, which exhibit much more decided 

 structural resemblances to birds than are presented by 

 any modern reptiles. One of the most remarkable of 

 these resemblances occurs in the bones of the hind-leg. in 

 which the upper i>art of the ankle, or tarsus, tends to unite 

 itself with the leg-bone, or tibia, thus simulating the 

 tibio-tarsns, or leg-bone of a bird. On the other hand the 

 lower half of the tarsus displays an equally marked 



' Cftmliridijo Natunil History— .Vmplnbia and Reptilee," p. 5. 



