II FLIES WITH AQUATIC LARV^ 131 



skin is thick, and the respiratory surface limited, or 

 they are inclosed in a shell, or they burrow in earth 

 or mud. 



We might expect to find that haemoglobin would 

 always be developed in the blood of animals whose 

 respiration is rendered difficult in any of these ways, 

 but any such expectation would prove to be un- 

 founded, and there are many animals whose mode of 

 life renders it necessary that oxygen should be stored 

 and economically used, which contain no haemoglobin 

 in their blood. Hence, while we have a tolerably 

 satisfactory reason for the occurrence of haemoglobin 

 in a number of animals whose respiratory surface is 

 limited, and whose surroundings make it a matter of 

 difficulty to procure a sufficient supply of oxygen, we 

 have to admit that many similar animals under the 

 same conditions manage perfectly well without hsemo- 

 globin. Such admission is not a logical refutation of 

 the explanation. I might fairly put forward the 

 baldness of mankind as at least the principal reason 

 for wearing wigs, and this explanation would not be 

 impaired by any number of cases of bald men who do 

 not wear wigs. The fact is that the respiratory needs, 

 even of closely allied animals, vary greatly, and 

 further, there are more ways than one of acquiring 

 and storing up oxygen in their bodies. 



Either the storage-capacity for oxygen of the 

 Chironomus larva is considerable, or it must be 

 used very carefully, for the animal can subsist long 

 without a fresh supply. I took a flask of distilled 

 water, boiled it for three-quarters of an hour, closed 

 it tight with an india-rubber bung, and left it to cool 



K 2 



