14 LECTUEES ON BIOLOGY 



But it can be proved biogenetically and phylogenetically that most 

 of these animals have descended from five-fingered ancestors, or 

 at any rate from such as had more than one finger. At an 

 early stage of incubation of the embryo of the penguin we can 

 readily distinguish three well-developed fingers and the rudiments 

 (Anlagen) of a fourth. The Archseopteryx of the Jurassic period 

 when fully developed had three movable fingers. Let us 

 endeavour to make clear the radical influence of changed condi- 

 tions of life by the consideration of a few prominent instances. 



If we observe a fish in its natural element and notice 

 how perfectly adapted are its fins to life and movement in the 

 water we would hardly think it possible that these specially 

 developed organs could be used for other than swimming actions. 

 Yet we know fishes who, with the aid of their fins, are not only 

 able to run, climb, and jump, but even to traverse not inconsider- 

 able distances flying. It was, of course, necessary for the fins 

 of these privileged animals to undergo extensive changes to 

 qualify them for their different functions. 



In fishmongers' shops one may now and then see a very 

 quaint and awkward-looking fish, the Sapphirine Gurnard or 

 Tub-fish (Trigla hirundo). It is from 40 to 60 cm. long and a 

 well-known inhabitant of the North Sea. It has a rounded body 

 and a thick, square head, protected by an armour of rough skin. 

 When touched it makes a peculiar noise which reminds one 

 of the creaking of a badly-oiled door, by rubbing together its 

 hard gill-covers. We know that the fins of fishes are supported 

 by cartilaginous rays which are generally firmly joined to the fin. 

 But in the gurnard the strongly developed pectoral fins have in 

 front three free, fin-like rays which seem fixed to the body after 

 the manner of joints. In swimming, these free fin-rays are 

 folded backwards against the body, but on the sea-floor the fish 

 uses them skilfully as legs and is thus able to walk. It is a 

 curious sight to see it deliberately putting one ray before the 

 other and drawing its plump body slowly along the sea-floor when 

 in search of its prey. 



A near relative of the gurnard is the Flying Gurnard (Dactyl- 

 opterus volitans), abundant in the Mediterranean, and a repre- 

 sentative of the flying-fishes. The structure of this fish reminds 

 one strongly of his cousin of the North, but is much smaller, at 



