24 DARWINISM AND EVOLUTION. 



composed of numerous horny vesicles filled with air, to the under 

 surface of which the animal attaches its eggs, and thus floats about 

 on the surface of the Atlantic in great numbers. 



In early tertiary times, when the ancestors of the seals took to 

 the sea, whales had already become entirely pelagic. Some seals, 

 the Otaridae, have four limbs on which they can support them- 

 selves. The hind feet, however, appear only as flappers, while 

 the corresponding legs are united with the tail by the general 

 integument of the body. In the common seal, phoca vitulina, the 

 front limb is buried beyond the elbow in the general integument, 

 but the flexible wrist allows the weight of the body to be sup- 

 ported by the palm. The hind limbs, on the contrary, are per- 

 manently stretched out and turned backwards, under the skin of 

 the trunk, so that the conjoined hind limbs and tail are put to 

 the same use as the caudal fin of a cetacean. 



The whale, which is as much a mammal as a cow, has its limbs 

 modified even more than those of the seal. Its arm-bones are 

 short, laterally compressed, and without movement upon one 

 another. The fingers do not exceed five in number, but some of 

 these always have more than three phalanges (knuckle-bones), 

 constituting a fin. The hind limbs are in most whales entirely 

 absent, but in the Balsenoidea they occur in the highly rudimentary 

 form of ossicles, or small bones, which lie on the outer side of the 

 pelvic plates. 



So much for rudiments ; though one highly important fact 

 remains to be stated. It is a rule without exception, that a 

 rudimentary part is of greater relative size in the embryo than in 

 the adult ; so that the organ at this early stage is less rudimentary, 

 or even cannot be said to be in any degree rudimentary. Hence 

 rudimentary organs in the adult are often said to have retained 

 their embryonic condition. 



The young of an animal, prior to birth, or before leaving the 

 egg, or previous to attaining the mature form, is an embryo. An 

 embryo is an animal in its foetal state. Larva? are active embryoes. 



6. Embryology, or the history of the germ, is the history of 

 Descent. In other words, the series of forms through which the 

 individual organism passes during its progress from the egg-cell to 

 its fully-developed state, is a representation of the long series of 

 forms through which the ancestors of that organism have passed, 

 from the earliest periods down to the present time. 



In the majority of ruminants, or cud-chewers, the lower jaw 

 alone is furnished with incisors or front teeth ; the only exceptions 

 being the camels. Now the foetal calf has incisors also in the 

 upper jaw ; but they never cut through the gum. Hence, we are 

 confident that the calf s distant ancestors had incisors in both 

 jaws. We draw a similar inference as to the ancestors of the 

 whale, from the fact of the presence of teeth in the embryo, 



