32 THE AGE OF THE EARTH 



leads to this conviction is based upon the fact that animal 

 structure and mode of development can be, and have 

 been, handed down from generation to generation from 

 a period far more remote than that which is represented 

 by the earliest fossils ; that fundamental facts in structure 

 and development may remain changeless amid endless 

 changes of a more general character; that especially 

 favourable conditions have preserved ancestral forms 

 comparatively unchanged. Working upon this material, 

 comparative anatomy and embryology can reconstruct 

 for us the general aspects of a history which took place 

 long before the Cambrian rocks were deposited. This 

 line of reasoning may appear very speculative and un- 

 sound, and it may easily become so when pressed too 

 far. But applied with due caution and reserve, it may 

 be trusted to supply us with an immense amount of 

 valuable information which cannot be obtained in any 

 other way. Furthermore, it is capable of standing the 

 very true and searching test supplied by the verification 

 of predictions made on its authority. Many facts taken 

 together lead the zoologist to believe that A was 

 descended from C through B ; but if this be true, B 

 should possess certain characters which are not known 

 to belong to it. Under the inspiration of hypothesis 

 a more searching investigation is made, and the charac- 

 ters are found. Again, that relatively small amount 

 of the whole scheme of animal evolution which is con- 

 tained in the fossiliferous rocks has furnished abundant 

 confirmation of the validity of the zoologist's method. 

 The comparative anatomy of the higher Vertebrate 

 Classes leads the zoologist to believe that the toothless 

 beak and the fused caudal vertebrae of a bird were not 

 ancestral characters, but were at some time derived from 

 a condition more conformable to the general plan of 

 vertebrate construction, and especially to that of reptiles. 

 Numerous secondary fossils prove to us that the birds 

 of that time possessed teeth and separate caudal vertebrae, 

 culminating in the long lizard-like tail of Archaeopteryx. 

 Prediction and confirmation of this kind, both zoo- 

 logical and palaeontological, have been going on ever 



