2 30 



NATURE 



\7an. 6, 1 88 1 



Thus I regard the Amphibian type as the representative 

 of the next lower stage of vertebrate evolution ; and it is 

 extremely interesting to observe that even the existing 

 Amphibia present us with almost every degree of modi- 

 fication of the type, from such forms as the oviparous, 

 branchiate, small-lunged Sircdoii and MenobraiiLhits, 

 which stand in the same relation to it as Gyminira to 

 the Eutheria, to the exclusively air-breathing Salaman- 

 ders and Frogs, in which the period of intraovular deve- 

 lopment, either within the uterus itself or in special 

 receptacles, may be as much prolonged as it is in the 

 Mammalia. 



A careful study, on full materials, of the development 

 of the young of such forms as Hylodes will probably 

 throw great light on the nature of the changes which 

 ended in the suppression of the branchiae, and the deve- 

 lopment of the amnion and of the extra-abdominal part 

 of the allantois in the fcetus of the higher Vertebrata. 



The recent researches of Boas ' on the structure of the 

 heart and the origin of the pulmonary arteries of Cera- 

 todus fell into my hands when I happened to be working 

 afresh at the subject, and had arrived, so far as the heart 

 is concerned, at results which are entirely confirmatory of 



his. This wonderful creature seems contrived for the 

 illustration of the doctrine of evolution. Equally good 

 arguments might be adduced for the assertion that it is 

 an amphibian or a fish, or both, or neither— the reason 

 of this being that, as it appears to me, Ccratodus is an 

 extraordinarily little modified representative of that parti- 

 cular stage of vertebrate evolution of which both the 

 typical Fishes and the typical Amphibia are special 

 modifications. I think it will be convenient to have a 

 name for the representatives of this stage, and I propose 

 that of Herpetichthyes. 



If we were to take away from Ceratodus the membrane- 

 bones of the heart and the pneumatocoele, and slightly 

 simplify the structure of the heart, the result would be an 

 animal which would undoubtedly be classed among the 

 Chiineroidei ; and if, in such a Chimseroid, the lamellar 

 septa of the branchiae were not reduced, as they are 

 in the Chimaroidci, while the opercular fold remained 

 undeveloped, the product would be a little modified 

 representative of the Selachian group, to which, among 

 actually known forms, Heptaiichus and Cestracion present 

 the nearest approximations. V^ertebrated animals in this 

 stage of evolution may be termed Chondrichthyes. 



Ma 



V Primates. 



deciduate. O 



Metatheria 



1. Teats. 



2. Allantoic placenta. 



3. Ureteric apertures ento- 



cystic. 



4. Small malleus. 



5. Reduced coracoid. 



6. Epipubis rudimentary I pi^^gnt^ 



or absent. ' 



7. Two occipital 



occipital. 



8. Amnion present, 

 g. A corpus callosun 



10. No branchise. 



I, 3. 4. S. 7. 8, 9, 



'IDEA. VORA 



above. 



/ 7, 8, 9, 10 as above. 

 i. No teats. 



ii. No allantoic placenta. 

 Prototheria iii. Ureteric apeitures hypo^ 



I cystic. 



iv. Large malleus. 

 V. Complete coracoid. 

 vi. Large epipubes. 

 r7.8.9,i..ii.,i.i.,iv.v.,vi. 

 as above. 

 Hypotheria. Jfl. No mammary gland. r^ r^ ^ 



\b. Mandible articulating r ^ " 



with quadrate. 

 [c. No corpus callosum. / 



Suppose the limbs and the genital ducts of the Chond- 

 richthyes-s\.a.gQ to be undeveloped, and let the two nasal 

 sacs be represented by a partially divided sac with a single 

 external aperture, the result will be a still lower grade of 

 vertebrate life, which may be termed Myzichihyes, repre- 

 sented only by the greatly modified Lampreys and Hags 

 of the existing fauna. 



Finally, let the head retain its primitive segmentation, 

 and the heart its primitive character of a contractile tube, 

 and we reach, in the Hypiclithycs. a stage of simplification 

 of the vertebrate type, from which it would be difficult to 

 remove any essential feature without reaching a point at 

 which it is questionable whether an animal should be 

 called "vertebrate." This stage is at present repre- 

 sented only by a singularly modified form, the living 

 AmphioxHs. 



Thus, in the order of evolution all the Vertebrata 

 hitherto considered may be arranged in nine stages : — i, 

 that of the Hypichlliycs'; 2, that of the Myzichthyes ; 3, that 

 of the ChoHdrichtliyes ; 4, that of the Herpetichthyes ; 5, 

 that of the Amphibia; 6, that of the Hypotheria ; 7, that 

 of the Prototheria; 8, that of the Metatheria; and, 9 



* " Ueber Herz und Arterienbogen bei Ccrntothts und Protopteyus" 

 Merpk. Jahrbiicli, 1880. 



that of the Eutheria. All these stages, except that of the 

 Hypotheria, are represented by existing groups of verte- 

 brated animals, which, in most cases, are composed of 

 greatly modified forms of the type to which they belong, 

 only the Amphibia and the Eutheria exhibiting near 

 approximations to the unmodified type in some of their 

 existing members. 



It will be observed that I have omitted to mention the 

 Ganoid and the Teleostean Fishes and the Sauropsida. 

 I have done so because they appear to me to lie off the 

 main line of evolution — to represent, as it were, side 

 tracks starting from certain points of that line. The 

 Ganoidei and the Teleostei I conceive to stand in this 

 relation to the stage of the Herpetichthyes, and the 

 Sauropsida to the stage of the Amphibia. 



There is nothing, so far as I can see, in the organization 

 ot the Ganoid and Teleostean fishes which is not readily 

 explicable by the application of the law of evolution to 

 the Herpetichthyes. They may be interpreted as effects 

 of the excessive development, reduction, or coalescence 

 of the parts of a Herpetichthyan.' 



' That the heart of Butirinns affords a complete transition between the 

 characteristically Ganoid and characrerislically Teleostean heart, has re- 

 cently been proved by Boas (.I/o»//;ii/. fahrbuch, 1880). Thus the last rem- 

 nant of the supposed hiatus between the Ganoids and the Teleostean vanishes. 



