262 MOVEMENT 



area of wing is greatly reduced, the bird can fly no 

 longer in the ordinary way through the air; but it 

 can fly fairly well along the water, like penguins and 

 birds of similar nature. This water-flight may be 

 observed also among certain marine tortoises, the lower 

 limbs of which resemble in shape the wings of a 

 penguin. Further, tortoises and birds have many 

 anatomical resemblances. 



The relation between the shape of the wing and 

 the character of flight can be traced as regards other 

 structural details. We have already shown that the 

 volume and shape of the muscles present differences 

 amongst birds which use their wings as sails and those 

 which use them as oars ; in the former the muscles 

 are short and thick, in the latter long and slender. 

 Similar differences are to be noticed in the bones of 

 the thorax which are characteristically grooved for the 

 insertion of these muscles. 



The differences in flight among the principal types of 

 birds, as far as we have been able to study them, have 

 been demonstrated by means of chronophotography, 

 and it is extremely probable that, by studying a great 

 number of species by the same method, we shall find 

 among the various types of flight, gradations and 

 transitions parallel to those anatomical variations 

 which have been recognized by comparative anatomists 

 among ornithological genera and species. 



Classification of Different Types of Locomotion. — It is 

 not always possible to take comparative anatomy as 

 a guide to physiological classification. Sometimes it 

 is the variation or functional analogy which may be 

 most apparent, and which will assist us in discovering 

 the zoological relationship. For this reason we have 

 endeavoured to procure photographs of a great number 

 of different species of animals, so as to collate the 

 various physiological types and classify them like the 



