204 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



structure, would not, as it would seem, be of any universal 

 interest ; for it must be very difficult, if not actually impossible, 

 to make experiments on such creatures. We must remain 

 satisfied with the fact that permanently fixed animals, or such 

 as can attach themselves temporarily, exhibit, without exception, 

 organs of such structure, form, and character as most clearly 

 prove the adaptation of the animal to the development; of its 

 powers of resistance. Their fitness to live depends exclusively 

 on this power of becoming fixed or of clinging ; and if once they 

 are displaced by a too violent lateral shock, death almost always 

 ensues. 



This group of sedentary animals, spending their lives fixed 

 to one spot, exposed to and resisting the pressure of water in 

 motion, may be contrasted with another which, to express 

 myself for once in the terms of the Teleologist, so far over- 

 come the momentum of the water as to use it to their own 

 profit. Swimming animals do not feel the strength of the 

 current so long as they swim with it or are carried on by it ; it 

 is only when they try to swim against it, directly or at an 

 angle, that they fee} its force, and it is not till then that they 

 develope those organs or characters from which they derive the 

 requisite power of resistance. It is perfectly immaterial to all 

 the very tender sea creatures, containing not more than from two 

 to three percent, of dry matter such as Siphonophora, Medusae, 

 Salpae, and larvae of all kind whether they are carried by the 

 current at a rate of two or of ten miles an hour ; nor, indeed, 

 can they be conscious of it so long as they are not flung against 

 a hard object. 



We will consider these two groups, and the organs which 

 distinguish and demarcate them, somewhat more closely. 



The organs and characters which are possessed by migratory 

 animals, and allow of their moving and of their taking advan- 

 tage of the transporting power of the currents, are extraordinarily 

 various ; and yet they admit of our easily dividing the animals 

 t into two great groups. These arc (a) the animals that move 

 I voluntarily or swim, and that move from place to place of their 

 own free will ; and (b) those that move involuntarily or float 

 and are carried passively by the stream. The former require 



