LECTURES ON EMBRYOLOGY. 



103 



the class of Reptiles; then come the Birds and Mam- 

 malia. The class of Fishes, which I have studied 

 more particularly, has shown me that the first 

 types appeared under forms and with an organi- 

 zation peculiar to embryos of that very class in the 

 present epoch, proving thereby with perfect evi- 

 dence the inferiority of the first created types, as 

 well in their peculiar class as in their department. 

 But though of a lower order, these types of an- 

 cient ages bore in themselves, from the beginning, 

 the impression of the plan that was to be succes- 

 sively developed in the different epochs which have 

 preceded the order of things existing at present, 

 and by whose realization have been brought about 

 those numerous families of Fishes, Reptiles, Birds 

 and Mammalia which live now on the surface of 

 the earth. According to this plan, a certain num- 

 ber of families were to be extinguished before our 

 epoch ; these families are known to us only through 

 their fossil remains, which researches in the crust 

 of our globe have brought to light. Other fami- 

 lies, less numerous, have lived through all the rev- 

 olutions of the globe, and have preserved some 

 representatives, a sort of reminiscence of a past 

 order of things, confined upon a few spots of the 

 present surface of the earth. 



It is worth while to notice that Northern Amer- 

 ica is the present home of several of those ancient 

 types. Such are, in the class of Fishes, the Lepi- 

 dostei, which perpetuate the order of Ganoids, in 

 our days, an order so numerous in the fossilif erous 

 strata of a former world, and the genus Percopsis 

 of Lake Superior, which represents a family which 

 prevailed in ancient times in Central Europe, dur- 

 ing the epoch of the deposition of the chalk. We 

 observe the same relation among the trees of 

 Northern America, which resemble much more the 

 vegetation of the tertiary period than the trees now 

 growing in Europe. 



The time has past which was allowed for this 

 course. I must come to some conclusions without 

 giving any further details upon the subj ect. 



My object has been to bring the present knowl- 

 edge which is possessed upon Embryology, into 

 one point of view. If I have succeeded in show- 

 ing that there is a common development to all 

 animals, however diversified, I have succeeded in 

 illustrating, perhaps, in a more philosophical view, 

 the different data which have been acquired upon 

 this point. All animals arise from uniform eggs, 

 however different their final development may be. 

 But however like they are at first, we soon notice 

 the difference. The growth of the germ in Radi- 

 ata does not take place in the seme manner as 

 that of Mollusca ; nor does it take place as in 

 Articulata ; and we have again seen that the growth 

 of the germ in Vertebrata takes place in a different 

 manner. And to make this more prominent by 

 figures,we can represent the Vertebrata,as we have 

 done with the other great types, as follows: 

 by a double crescent in two opposite directions, 

 showing that there is a special cavity containing 



the brain and the main organs of sensation, and a 

 lower cavity containing the intestines and respira- 

 tory organs. And this symbol will be only a copy 

 of the outlines of the embryonic growth of any of 

 these vertebrated animals. 



But we have found these metamorphoses to 

 agree in many instances with the gradation which 

 structure had illustrated. We may therefore in- 

 fer from the successive development of structure, 

 the order in which animals should follow in a 

 natural arrangement, as ascertained by the 

 knowledge of metamorphosis. So that, vice versa, 

 the study of Embryology will improve our classi- 

 fication, as derived from anatomical data, as well 

 as anatomical investigation will go to complete 

 the inferences derived from Embryology. 



I think I have particularly been able to show 

 that classification in its details may be improved 

 by Embryological evidence. And it is upon this 

 point I would insist : that a more extensive knowl- 

 edge of young animals will be extensively useful 

 to the further progress of Zoology, as affording, by 

 the comparison of successive changes, the means 

 to assign to full grown animals their respective 

 places in any given group. 



The facts are already numerous enough to allow 

 us to consider this principle as the fundamental 

 principle of classification, which should overrule 

 the information derived from Anatomy in the de- 

 tails, as here Embryology assigns a value to the 

 external forms for which comparative Anatomy has 

 no understanding. Comparative Anatomy has not 

 been able to value the external forms, to assign to 

 them any importance. But Embryology, by the 

 metamorphoses which take place in animals, as- 

 signs now a value to external forms, and not only 

 assigns to them a value, but a chronological value, 

 by which it is possible to consider as lower those 

 animals which agree with the earlier forms of the 

 germs. 



These remarks would lead me to make some ob- 

 servations upon what is next to be done in these 

 investigations. That a greater number of animals 

 must be investigated than has been done before, is 

 obvious. There are several animals, upon which 

 we have no information. 



But these results should not be traced simply 

 with reference to Physiology, as it has been hither- 

 to. All Physiologists have traced them with refer- 

 ence to the structure of the organs, to the structure 

 of the tissues, to the structure of their various 

 systems, and not with a view to understand their 

 forms. Simple sketches of the outlines of various 

 forms of germs from various families,with their de- 

 scription, would be a highly valuable contribution 

 to the stock of our knowledge at present, and would 

 afford, as rough as thev may be, the means to place 

 in a natural position many genera which are now 

 placed in a most arbitrary order in our method. 



I do not undervalue our past labors in classifica- 

 tion, but I make a distinction between what has 

 been done in an arbitrary manner, and what has 



