Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III. SWIMMING ANIMALS. In the preceding chapter we discussed the various structural modifications by means of which the members of different groups of animals are enabled to fly, or, in other words, to swim in the aerial ocean. From the observations there recorded, it is evident that all the creatures adapted for this peculiar mode of life have been specially modified for that purpose; flight thus always being a power which has been specially acquired, and not one which was an original attribute of any group of animals. It is our purpose in the present essay to notice in a somewhat similar manner the various adaptations of the structure of certain animals whereby they are enabled to swim in the denser medium of water. And here we shall find that while there is conclusive evidence to show that in many instances this power is an acquired one, yet there are others which lead to the belief that in certain groups it is a primitive function. Some clue as to the groups in which this power of swimming is an acquired one, and those inwhich it is a primitive one, is afforded by the different modes in which aquatic animals breathe. Thus in fishes the air necessary to oxygenate the blood is obtained from that dissolved in the water itself by its constant passage over those peculiar comb-like organs, highly charged with blood, known as gills; such animals having, therefore, no occasion to come to the surface of the water to breathe. In other animals, however, such as Whales and Grampus (Fig. 14), atmospheric air is breathed directly by means of lungs, necessitating visits at longer or shorter intervals to the surface, and it is in such instances that we may safely infer that the adaptation to an aquatic life has been gradually developed from ancestors whose normal habits were te...