370 ADAPTATIONS TO MEDIA AND SUBSTRATES 



When the two sides of a Hving membrane are bathed by liquids con- 

 taining differing amounts of dissolved substances, there is a net flow of 

 water through the membrane toward the side of the higher concentration. 

 The easy way for a living cell, or a whole organism, to maintain its 

 water content is to take advantage of this fact and expose its salty proto- 

 plasm to plain water, with a semi-permeable membrane intervening. A 

 freshwater organism therefore need not drink water, for plenty of it is 

 penetrating his surface continuously under the drive of osmosis. But in 

 getting his water in this way the animal is literally playing with explo- 

 sives. Unless controlled in some way (usually by excreting water about 

 as fast as it comes in) the pressure built up inside the cells by osmosis 

 will carry them past the desirable degree of turgidity, and will burst 

 them. The freshwater organism has this problem of osmosis-control more 

 urgently than the marine organism, for the disparity of concentrations 

 of dissolved substances within and outside the body is far greater. More- 

 over, the freshwater form must provide against the loss of essential salts 

 in the excreted water. We know, however, that the ancient seas were far 

 less salty. While it is often said that the vertebrate blood was originally 

 entrapped sea-water (the concentrations of salts in human blood there- 

 fore supposedly portraying the chemical pattern of the Devonian ocean) 

 we may be sure that the protoplasms of marine animals of the remote past 

 had higher osmotic pressures than that of the water outside — it would 

 have been disastrous for protoplasm to have become otherwise. There is 

 considerable evidence that the first vertebrates arose in fresh water, and 

 we can be quite sure that the land animals evolved from freshwater fishes. 

 Even at the present time it is the marine fish, not the freshwater one, 

 which exhibits special devices in connection with the control of water- 

 balance. 



The rigidity of the vertebrate eye, which makes it a good optical 

 instrument despite its constitution from soft and flexible tissues, is due 

 to a bit of hydraulic trickery. The eye owes its firmness to the fact that it 

 has a flexible but inelastic capsule which is kept distended by fluid pres- 

 sure. The same principle makes a hollow tennis ball just as firm as a 

 solid handball, and would allow us, if it were necessary, to put mobile 

 objects inside the tennis ball — a point which will seem important in a 

 moment. 



The primeval source of this distensive intra-ocular pressure was osmosis. 

 Some excellent invertebrate eyes (as in some cephalopod molluscs) have 

 employed this force in the same way. In others, such as the compound 



