394 HISTORY OF SCIENCE. 



degradation in organization may be crossed, confused, and concealed 

 by the modifications of organization which adapt animals to different 

 habits of life. Taking the Mammalia, he places the hoofed vegetable- 

 eating mammals below the digitated or carnivorous, and above the 

 Cetacea (whales, etc.), which have only one pair of limbs, with the digits 

 not externally separated, and without masticating organs. Although 

 the Amphibia as a class are below the Mammalia, it does not follow 

 that all the Amphibia are beneath the lowest of the Mammalia in 

 organization. Birds, by the organization of the heart, warm blood, 

 and some other important circumstances, constitute the class ranking 

 immediately below Mammalia; but they exhibit a marked degradation 

 in being oviparous. The diaphragm dividing the chest from the ab- 

 domen, and the mammce, which are wanting in the birds, do not re- 

 appear in any lower class. Birds retain a very important feature of 

 the circulatory system of the higher classes, inasmuch as they have a 

 complete pulmonary circulation, the whole of their blood passing 

 through the lungs, and this arrangement is not found again in any 

 lower class. Reptiles resemble all the classes below them in being 

 cold-blooded. They have a heart with only one ventricle, and are the 

 last class in the series which breathes by lungs. The respiratory organs 

 of all the classes below are never in a proper sense lungs. The lungs 

 of reptiles are, however, much simpler than those of the animals above 

 them, and only a portion of the blood passes through them. Some 

 species breathe by lungs only in the mature state, and others are with- 

 out the limbs which all other Vertebrates possess. The lowest class 

 of Vertebrates is the Fishes, not because their outward form is so 

 different from other Vertebrates, but by reason of their inner organi- 

 zation. Their respiration is aquatic, and performed by means of the 

 bronchia (or gills). The oxygen necessary to respiration is derived 

 from air dissolved in the water, and the water is taken by fishes into 

 the mouth in order to reach the respiratory organ. Fishes are the 

 lowest class of animals which breathe by the mouth. Their skeleton 

 may be called a rudimentary sketch of that found in the higher ani- 

 mals ; the heart has only one ventricle ; the brain is very small. In 

 passing down the scale, organs which are of the greatest importance 

 with higher animals successively disappear. The eyes become simpler 

 in structure, and then cease to be found at all ; the nervous system 

 becomes continually less complex, until at length mere traces of its 

 existence are all that can be distinguished. The head, which is dis- 

 tinct enough in some of the higher classes of Invertebrates, ceases to 

 be found in the lower. We come at length to creatures in which almost 

 the only organ is a stomach or cavity for the reception of food. Finally 

 we reach the confines of the animal kingdom in minute, transparent, 

 gelatinous bodies of apparently uniform structure throughout, with 

 little consistence, yet exhibiting the irratibility which characterizes 

 animal substance, contractile, receiving nourishment from the liquids 



