102 



PROF. AGASS1Z S 



[PLATE X LEGS OF MAMMALS, BIRDS AND REP- 



'TILES] 



Here is the bind leg of an adult Squirrel, (Plate 

 X, fig. A), and here it is in an earlier stage of de- 

 velopment, (Fig. D) Here is the foot of a Robin 

 (Plate X, fig. B) with its covering, scales and claws ; 

 and here is the foot of a Robin at an earlier period, 

 (Fig. E). In the Frog here, it is with distinct fin- 

 gers, (Fig. C) and in the earlier periods still more 

 different, (Fig. F). 



What do we observe here ? Animals which 

 have so different organs of locomotion as the foot 

 of a Squirrel, or the foot of a Bird, or the foot of a 

 Frog, all have the same form originally. We may 

 therefore say that the organs of locomotion, how- 

 ever diversified in their perfect state, begin under 

 the same form. Could I have illustrated all those 

 which I have already traced, I should have shown 

 that the wing of a Robin is also in the beginning a 

 fin. And indeed, in all animals, which I have 

 traced with reference to these transformations of 

 the young, I have found that at the first period of 

 appearance,their organs of locomotion are through- 

 out identical in the Swallow as well as in the Spar- 

 row, the Wading Birds, the Birds of Prey, and in the 

 Swimming Birds, in all the legs and wings in their 

 earlier condition. The legs and wings are similar to 

 the lowest appendages or fins which we observe in 

 Fishes, showing also that even in the first outlines 

 of their organs there is the greatest uniformity. I 

 may say the same with regard to the structure of 

 the heart, of the first formation of the lungs, which 



are so complicated in Mammalia. It is a simple 

 sac in young Birds, and a simple sac in Fishes, 

 which have also that organ in a rudimentary con- 

 dition, as an air bladder, just to preserve the uni- 

 form harmony which exists in all these classes. It 

 is obvious what important consequences such facts 

 must have for the study of Zoology. They show 

 at once that all animals of any given group which 

 have webbed organs of locomotion, must be con- 

 sidered lower in their group than those in which, 

 this apparatus has become free has grown inde- 

 pendent. 



To make direct application of these results to 

 the classification of birds ; let me allude to the 

 fact, that at the present day, in our classification of 

 birds, all the birds in which the toes are united by 

 a web, are combined into one group however dif- 

 ferent they may be in the structure of their wings 

 or feathers, in the development of their inner or- 

 gans, or in their mode of living. We have, in fact, 

 in that group, the lowest birds of various types 

 united into one very unnatural family as we have 

 there brought together the Penguin, having imper- 

 fect wings used as fins, with the fleetest birds. 

 If we compare the bills, the predatory habits of 

 some, and the low mode of living of others, if we 

 consider the difference between the bills of Ducks 

 and of the Gulls, we may conclude, after the facts 

 above mentioned have been once ascertained, that 

 it is likely we have combined in one group the low- 

 est types of various families, which should be se- 

 parated ; and it is probable that we shall soon see 

 a re-arrangement of the class of birds, which will 

 be classified on other grounds, and leave in each 

 group some swimming types and some types with 

 divided fingers, which may be combined by some 

 higher characters. 



Already among the Vultures we have some re- 

 semblance to the Gulls, and it was only from their 

 curved claws and hooked bills that they could not 

 be brought together. But when I mention that 

 within the egg, the young Robin also has a hooked 

 bill, we see that the difference between the birds of 

 prey and the web-footed Gulls cannot be so very 

 great particularly when we notice the appear- 

 ance of the joints and fingers in their embryologi- 

 cal formation, and the fact that so many birds of 

 prey have the outer finger united by a rudimen- 

 tary web. Without entering into minute details, I 

 may state that the knowledge of the changes 

 which each germ undergoes to assume the form of 

 the full grown individuals, will obtain for us a 

 complete key to the natural arrangement of the 

 perfect animals. 



Contrary to what happens in the Radiata, Mol- 

 lusca and Articulata, where all classes appear sim- 

 ultaneously from the first times of development of 

 animal life, the four classes of Vertebrata appear 

 successively in different epochs of the development 

 of our globe,in the order of gradation which Anat- 

 omy and Embryology assign to them. The class of 

 fishes appears first-, it is followed in a later epoch by 



