162 LECTUEES ON BIOLOGY 



it up. The bird which is looking for a fiery-red insect is all the 

 more apt to overlook the insignificant sand-coloured animal which 

 cowers motionless on the ground. For when the locust is in a 

 resting attitude the underwings are completely covered by the 

 upperwings. It seems, therefore, that the striking colour is only 

 one more protective measure for these insects. It might be 

 possible to explain similarly the striking colour of the under- 

 wings of the death's-head moth, some 'owls,' 'red underwings/ 

 and many other butterflies. 



It is further known that the inhabitants of grass and 

 leaves, such as grass-locusts, many caterpillars and larvae, 

 frogs, lizards, etc., are green. Only birds appear to supply the 

 exception to this rule. There are no green species of birds in 

 Europe, with the exception of the green woodpecker, greenfinch, 

 siskin, serin, some tomtits, the golden-crested wren, and a few 

 others. But if we consider that in our climate trees and shrubs 

 are without leaves during a great part of the year, we shall 

 understand that here a green colour gives to birds only a very 

 doubtful advantage. On the other hand, in the Tropics, or in 

 countries with evergreen forests, we meet very frequently with 

 green birds. 



As regards the denizens of the water, we became acquainted 

 in the last lecture with the dainty Leptodora hyalina, a little 

 freshwater crustacean, whose transparent body is almost 

 invisible. But the phenomenon observed in Leptodora is not 

 an isolated one. Numerous other inhabitants of fresh water and 

 the sea have acquired a similar transparency. Fixed to the 

 ocean-floor live the " glass-rope sponges." Who would regard 

 these beautiful forms as animals ? Do they not rather seem 

 like artistically executed Venetian crystal ornaments woven 

 from the finest glass threads ? Floating about the ocean are 

 immense numbers of salpae, medusas, jelly-fishes, snails and 

 crustaceans, swimming in close formation through the water, 

 yet being hardly visible. Even the eggs and free-swimming 

 larvae of many of the lower marine forms are distinguished by 

 perfect transparency (see fig. 46). 



A remarkable phenomenon of adaptation must be briefly 

 mentioned. It is well known that light can only penetrate to 

 relatively shallow depths of the sea, perhaps 400 to 600 fathoms ; 



