CLASSIFICATION OF ANIMALS. 193 



retaining of the gills indicates a lower position than their disap- 

 pearance. * 



Of course, in the outset, we do not find sufficient data to trace this 

 arrangement throughout the animal kingdom, and to make the prin- 

 ciple which I have just mentioned the ruling law of nice classical 

 arrangement. But until such sufficient knowledge is acquired, let me 

 show that my principle does in fact apply to all classes of the animal 

 kingdom, and will at once contribute to improve all their subdivisions. 

 Among Mammalia, for example, we shall continue to give the aquatic 

 carnivorous animals a lower position among Carnivora, but no ouger 

 simply because they are aquatic, but because they are webfooted, as 

 the webfoot is the earlier form of the limbs in all Mammalia whose 

 embryonic development has been traced. We shall be led, for similar 

 reasons, to deny the bats the high position which has been assigned 

 to them, and to combine them closer with the Insectivora. We shall 

 separate the manatees from their present relations and combine them 

 with tapirs, elephants, &c., as they are rather webfooted Pachy- 

 derms, than true Cetaceans. f 



* These views were fully illustrated in a series of twelve lectures upon Comparative 

 Embryology, delivered before the Lowell Institute during the last winter, and reported 

 for the Daily Evening Traveller, and afterwards published as a separate pamphlet. 



f These aphorisms will be justified by a more elaborate illustration of the peculiar 

 changes which the limbs of Mammalia undergo during their embryonic growth, as far 

 as I have been able to trace them, in various animals. It may suffice, for the present, 

 for me to say here, that in all young embryos of Mammalia which I have recently had 

 an opportunity to examine, I have found the extremities arising as oblong tubercles, 

 flattened at their extremities, spreading more and more into the form of hemispherical 

 paddles, in which the changes in the cellular growth gradually introduce differences 

 upon the points where the fingers are to be developed. But for a longer time they re- 

 main combined in a common outline, and the microscopic structure of the tissues alone 

 indicates the points of growth ; and even after the fingers have been fully sketched out, 

 they remain for a certain time united by a common web, which is successively reduced 

 as the fingers grow longer and thicker. 



It is very remarkable how uniform, and indeed how identical in form and structure the 

 anterior and posterior extremities are in the beginning, whatever may be the difference 

 at a later period of growth. Thus, for instance, there is not the slightest difference be- 

 tween the anterior and posterior extremities of the bat, in the early stages of develop- 

 ment. The wing is then a very short limb, terminated by a flat, webbed paddle, of a 

 semicircular form, identical in development, size and form with the hinder extremity, 

 and differing in no respect from the appearance of the hand and foot in young human 

 embryos, or in embryos of cats, dogs, squirrels, hares, rabbits and pigs, and bearing 



