INTRODUCTION 3 



of the embryo chick is not an ancestral character, but an 

 instrument developed for the purpose of cutting a way out 

 of the shell — it is a tin-opener, in short, and disappears soon 

 after hatching. But the ancestral bird Archesopteryx, we 

 know, had a long lizard-like tail, formed of a number of 

 short cylinders of bone, arranged chain-wise, each bearing 

 a pair of tail quills. In the adult bird to_rday we find the 

 tail quills arranged fan-wise, on either side of a short 

 bony blade, an absolutely different plan. But if we 

 examine the tail of an embryo bird, we shall find this blade 

 is made up of a number of separate pieces corresponding 

 with that in the row of bones in the ancestral bird. 

 Thus we find that, in course of ages, the tail has undergone 

 a process of " telescoping," the separate cylinders of bone 

 growing shorter and shorter, so that the tail feathers have 

 been brought closer and closer together till the tail of the 

 modern bird has come into being. 



Similarly, we know that the whalebone whales of to- 

 day are descended from tooth-bearing ancestors, for we 

 find in the jaws of the embryo, vestiges of teeth which 

 never cut the gum. And we might multiply instances of 

 this kind indefinitely from the records of the embryologist. 

 But we are concerned here with post-embryonic life : and 

 here we shall also find characters which on the one hand 

 are reminiscent of past conditions of existence, and on the 

 other characters which either belong to, or are peculiar 

 to, the infantile stages or are foreshadowings of what is 

 to be. 



Nowhere is this conflict between the present and the 

 past more fascinatingly presented than in the study of 

 the coloration of young animals, and on this theme we 

 shall have much. to say in these pages. But apart from 

 these purely physical considerations there are yet other 



