LECTURES ON EMBRYOLOGY. 



So much for the views referring to embryology 

 in its bearing upon zoological classification. 



There is, however, another field in which the 

 animal kingdom has been represented as developed 

 according to the gradation of its structure : I mean 

 the order of succession of extinct species in geolo- 

 gical times, It has been long and generally aa- 

 sertedj especially by the physio-philosophers, that 

 the lower animals were first introduced upon our 

 globe, and formed alone the population of the 

 earliest periods in past time; that Polypi existed 

 before Mollusks ; these before Articulata, and that 

 Vertebrata were the last to make their appear- 

 ance. But the discoveries in fossil Ichthyology, 

 which it has been my good fortune to describe in 

 my researches upon fossil fishes, have shown that 

 vertebrated animals, fishes, have existed in the 

 oldest epochs, and that such an order of succes- 

 sion, as mentioned before, did not agree with the 

 plan of creation. Indeed, that representatives of 

 all the four great divisions of the animal kingdom, 

 Articulata, Mollusca and Radiata, occur simulta- 

 neously with fishes, in all the lowest geological 

 formations, was soon ascertained by the investi- 

 gations of paleontologists, and the fact of any reg- 

 ular succession was afterwards altogether denied. 

 However, the simultaneous occurrence of the four 

 great types does not yet indicate the want of reg- 

 ularity in the development of the various classes 

 of the animal kingdom, taken isolately. Several 

 eminent paleontologists, Leopold Von Buch, Count 

 Von Murster, Sir R. Murchison, d'Orbigny, Prof. 

 James Hall, and many others, have shown that the 

 types of different classes which characterize the 

 different geological ages, follow each other in an 

 order which agrees with their zoological gradation 

 as ascertained by structural evidence. The great 

 difference between this fact and the views enter- 

 tained before, consists in the knowledge of the in- 

 dependent gradation of the different classes, which 

 in the lower types arise all simultaneously, to un- 

 dergo their metamorphoses simultaneously,through 

 all geological periods, whilst among Vertebrates, 

 the Fishes were found to occur earlier than Rep- 

 tiles, and these earlier than Birds and Mammalia, 

 which made their appearance last. It was in that 

 way shown that there is a progressive succession 

 of classes among Vertebrata, ending with the cre- 

 ation of Man ; whilst Polypi and Echinoderms 

 among Radiata; Acephala, Gasteropoda and Ceph- 

 alopoda among Mollusks; Worms, Insects and 

 Crustacea among Articulata, existed simultane- 

 ously during all great periods, and presented each 

 a development of its own. 



However, another step had to be made to show 

 a reaPagreement between the earlier types of an- 

 imals and the gradual development of the animal 

 kingdom, which has been the last progress in our 

 science of fossils: namely, to show that these ear- 

 lier types are embryonic in their character that 

 is to say, that they are not only lower in their 

 structure when compared with the animals now 



living upon the surface of our globe, but that they 

 actually correspond to the changes which embryos 

 of the same classes undergo during their growth. 

 This was first discovered among fishes, which I 

 have shown to present, in their earlier types, char- 

 acters which agree in many respects with the 

 changes which young fishes undergo within the 

 egg. Without entering i nto all the details of these 

 researches, I will concluc by saying, it can now be 

 generally maintained tha earlier animals corres- 

 pond not only to lower vpes of their respective 

 classes, but that their chief peculiarities have ref- 

 erence to the modifications which are successively 

 introduced during the embryonic life of their cor- 

 responding representatives in the present creation, 

 To carry out these results in detail must now be, 

 for years to came, the task of paleontological in- 

 vestigations. 



But the other connections mentioned above, I 

 consider as established, and I claim these views as 

 the results of my own investigation, though much 

 has already been said upon the natural and suc- 

 cessive development of the animal kingdom, and 

 upon the propriety of introducing a classification 

 based upon embryology, The views to which I 

 allude are indeed not the same as those which I 

 advocate ; and in order to avoid mistakes in this 

 respect, I will now dwell for a moment upon this- 

 point, with the hope, perhaps, to show that these 

 views are incorrect, and must be given up, though 

 they pretend to lead to a natural arrangement of 

 the animal kingdom. The first notion of progres- 

 sive development of the animal kingdom, of an 

 agreement between the order of succession of types 

 and their structural gradation, has been brought 

 forward by that school of philosophers who in 

 Germany take the name of nature-philosophers, 

 (physio-philosophers.) But with them the idea of 

 a gradual development of the animal kingdom, 

 was by no means the result of investigations was 

 not the expression of facts, but was an a priori 

 conception, in which they made their view of the 

 animal kingdom the foundation for a particular 

 classification, seeming also to agree with the little 

 that was known of geological succession of types. 

 Dr. Martin Barry, a distinguished physiologist 

 in London, has however proposed principles for 

 classification of the animal kingdom, which de- 

 serve more particular notice, as he presents them 

 as the results of his extensive investigations in 

 embryology, and he has put his view upon the 

 subject in the following words. Dr. Barry is one 

 of the ablest investigators in this department, one 

 of those who have most extensively studied the 

 egg and its developments in the mammalia. To 

 him and to Dr. Bischoff we are indebted for the 

 mosf elaborate investigations upon this subject ; 

 but I am not aware that Dr. Barry has traced the 

 metamorphoses of animals in other classes. His 

 views are substantially expressed in the following 

 statements : " There is no appreciable difference 

 in the germs of all animals. There is a fundamen- 



