INTRODUCTORY 5 



of reproduction. Similarly, the microscopic structure of the growing 

 feather in the embryo, or unhatched chick, shows that it is really an 



Femur 



Patella 



or 

 Knee=cap 



Tibia 



or 



Stiaak 



III 





Cannon=botte 



Fig. 2.-Diagram of the Leg of an Adult Bird, showing the Condition of the 

 Limb after the Separate Elements have become welded. 



extremely elaborate reptile scale, and is formed on a plan quite different 

 from the hairs of mammals. 



But, it may be urged, it is all very well, and it may be quite true, 

 to say that because the reptiles and the birds have so much in common, 

 therefore they must be related. But why should we assume this ? 



