9i8 LIFE : OUTLINES OF GENERAL BIOLOGY 



their climax in the Carboniferous. Then in the Mesozoic periods was 

 the prolonged Golden Age of Reptiles, from amongst which the 

 Birds and the Mammals gradually arose. The most general of all 

 palaeontographical facts is perhaps the most impressive as an 

 "evidence of evolution" — ^namely, the successive emergence of 

 nobler and finer forms of life. 



The palaeontological argument includes the occurrence of "con- 

 necting links", such as the first known bird, Archaeopteryx, from the 

 Jurassic, which, though feathered and in other ways distinctly 

 avian, was reptile-like in having teeth in its jaws, a long lizard-like 

 tail, a half-made wing (i.e. without a fused carpo-metacarpus) , 

 claws on three fingers, and such a convincing little detail as 



^ 



4 



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Fig. 156. 



Hint of Part of a Genealogical Tree in which there has been repeated dicho- 

 tomy or bifurcation, as indicated by the small twigs, which may represent 

 species or genera or families. The dotted portion indicates the absence 

 of annectant types in the lineage or pedigree. After Doderlein. 



"abdominal ribs", which recall those of crocodiles and many other 

 reptiles. 



Very striking also are the fossil series which link species to species, 

 as is so well illustrated by the Tertiary species of the freshwater 

 Snails Paludina and Planorbis. Form A is distinguishable at a glance 

 from form Z, but the gradualness of the series which connects them 

 is very remarkable. Thus in the Planorbis multiformis series there 

 is a strikingly smooth transition from a high spiral to a flatly coiled 

 disc (cf. Fig. 138). 



On a larger scale, the series of fossil horses, elephants, camels, and 

 Ammonites are convincing to all open-minded students. 



(c) Morphological. — It is one of the most instructive of bio- 

 logical lessons to compare a series of, say, fore-limbs in some detail 

 — e.g. of frog, turtle, crocodilian, bird, bat, horse, and man. 



