LECTURES ON EMBRYOLOGY. 



developed before the 3'oung animal assumes forms 

 related to genera to which it can never be refer- 

 red. Indeed, the tadpole has all the peculiar ap- 

 pearances of batrachians with permanent gills, be- 

 fore a frog can be recognized ; it resembles suc- 

 cessively Menobranchus, Triton, and Menopoma, 

 before it loses its tail : and as for toads, they have 

 webbed feet, that is to say, they resemble another 

 genus, the frogs, before their fingers are entirely 

 separated, though the species can be recognized in 

 the distribution of colors long before. 



(PLATE III FROGS ) 



Professor Mi lne<Ed wards, of the Jardin des 

 Plantes,has proposed similar views, and indeed ex- 

 pressed in nearly the same words, his conviction 

 about the gradation of the animal kingdom ; but 

 not with reference to the development of zoologi- 

 cal characters, but with reference to the changes 

 W hich the animals undergo in their structure. He 



has referred his views more particularly to the 

 structure and the development of the functions of 

 animal life; and from this circumstance his views 

 agree better to nature, when he says that those 

 organs are first developed which are more impor- 

 tant to life. However, strictly speaking, it is not 

 absolutely true. It is the nervous system which 

 we may consider as the organ most important to 

 life; and it is not the nervous system which becomes 

 first apparent in the embryonic changes. The sys 

 terns by which the body grows are developed be- 

 fore those by which it lives a higher life come into 

 play; so that, though in a general way, the organs 

 most important to existence are really developed 

 first, it cannot strictly be said that they are the 

 higher organs which are developed first; and that 

 the special differences which characterise families 

 and genera should be engrafted as it were upon a 

 fundamental plan. 



My aim is an entirely different one, as you may 

 have perceived from my first lecture. It is to show 

 that in the real changes which animals undergo 

 during their embryonic growth, in those external 

 transformations as well as in those structural mod- 

 ifications within the body, we have a natural scale 

 to measure the degree or the gradation of those 

 full grown animals which correspond in their ex- 

 ternal form and in their structure, to those various 

 degrees in the metamorphoses, and therefore to 

 make the metamorphoses of animals, as illustrated 

 by embryonic changes, a real foundation for zoolo- 

 gical classification. 



Let me only mention that on the whole, the high- 

 er families of the various classes of the animal king- 

 dom are distributed over the warmer parts of the 

 present surface of our globe, and that the lower 

 families are rather numerous in the milder and 

 colder regions. Thus among mammalia,the Mon- 

 keys are strictfy circumscribed within the limits of 

 the growth of Palm trees ; the large carnivorous 

 beasts prevail in the tropical regions; whilst the 

 sheep, goats, and oxen are natives of the temper- 

 ate zone ; among reptiles.the crocodiles occur only 

 in the warmest countries; whilst the lower Batra- 

 chians, those with external gills or permanent tail, 

 extend even far north. There are, nevertheless, 

 inferior families which are also strictly tropical; 

 such for instance as the Pachyderms, and to some 

 extent, the Edentata ; but this fact has doubtless 

 reference to the early introduction of these fami- 

 lies in the plan of the creation, during a period 

 when the surface of our globe was warmer than it 

 is iu our days; so that the location of their modern 

 representatives in the torrid zone, can be consid- 

 ered as merely determined by the peculiar adapta- 

 tion of their general plan of structure for warmer 

 climates, rather than related to the gradation of 

 the types, according to the present condition of the 

 distribution of heat upon our globe. The induce- 

 ment for their present location is not their higher 

 structure, but their relation to earlier types. 

 But now, I proceed to illustrate the history of 



