ORGANS FOR SWIMMING. 207 



have a direct reference to their purpose and function. In the first 

 place, they have cutting edges and a broad surface, by which 

 the animal is enabled to exert pressure on the water in the 

 most efficient and natural manner ; in the second place, they 

 are, without exception, attached by movable joints in such 

 a way as to serve for steering. Such fins occur in the greatest 

 variety in both Vertebrate and Invertebrate animals ; in whales, 

 sea-serpents, and tailed Amphibia, on the tail ; in many Fishes, 

 as single fins on the back, belly, and tail, and as paired 

 fins on the body, where the lower or hinder extremities 

 or limbs are modified into fins. In many Birds, both wings and 

 legs serve as fins ; 98 Mammalia and Reptiles alike are often 

 fin- or web-footed (crocodiles, turtles, and aquatic mammalia). 

 In insects, the legs (in Notonecta) and sometimes special ap- 

 pendages of the body (as in the larvae of Ephemera) or even 

 the wings (Polynema), are used for swimming ; in many Crusta- 

 ceans all the legs are true fins, and in several of the Annelida 

 each segment of the body bears a pair of fins. The larvae of 

 Mollusca have them on the head, Cattle-fish at the hinder end 

 of the abdomen ; in short, there is hardly any portion of the 

 body on which some little lobe or process might not serve as a 

 fin. A small and delicate creature, like a Medusa or the larva 

 of a Mollusc, may find a ciliated disc or margin, or even a few 

 scattered cilia, efficient as fins in spite of their fragility ; large 

 animals, as whales, sharks, <fcc., require larger fins provided 

 with strong muscles, and supported on an internal skeleton. 

 In all cases the serviceableness of these organs depends on their 

 being fitted to move the whole mass of the creature in a defi- 

 nite direction, with or against currents ; if the extent of sur- 

 face of the fins, or the strength of their motor muscles, or the 

 supporting power of the skeleton, is insufficient for this purpose, 

 the individual possessing such inefficient fins must necessarily 

 perish. Thousands or millions of such inefficiently equipped 

 aquatic animals must be swept away every day and every hour 

 by the currents to which they are forced to commit themselves, 

 and here again the external conditions of existence select the 

 stronger, and eliminate the weaker, individuals, without the 

 need of any personal struggle between them. 



