32 ANIMAL LIFE AND HUMAN PROGRESS 



Its place is" taken by more lowly members of the stock from 

 which it was originally derived. We see this has happened 

 again and again. One may give as examples the Ammonites 

 which were abundant in later Palaeozoic times, and in the 

 secondary period look as though they were striving to maintain 

 a foothold by ever-increasing elaboration of structure. But 

 they failed and became utterly extinct. The Reptiles under- 

 went an astonishing development in size and adaptation to 

 different modes of existence in the secondary period. They 

 were giants of the land and sea, and must be reckoned among 

 the most formidable animals that the world has ever seen. 

 They became denizens of the air as well as of the sea and 

 land. But all these terrifying monsters with their highly 

 developed protective and aggressive structures died out, and 

 though the class survives in large numbers and variety to the 

 present day, it has lost its former pre-eminence and has had to 

 yield pride of place to two of its offshoots, the birds and 

 mammals. 



Numerous, too, are the cases in which there has been no 

 ascent in the scale of organisation, but, contrariwise, a degrada- 

 tion, most obvious in' sedentary and parasitic animals. This 

 may go so far that many parasites are reduced to mere sacs, in 

 which all organs of perception, locomotion, respiration and 

 even alimentation have disappeared, and little more is left than 

 the reproductive organs. 



On the whole, however, the progress has been forwards and 

 not backwards, though the history of animal evolution teaches 

 us that the battle has not always been to the apparently strong 

 nor the race to the apparently swift. There have been many 

 ups and downs, many failures on the part of organisms that 

 seemed competent to overcome all their contemporaries in a 

 struggle for existence. On the other hand, many quite humble 

 and relatively lowly organised forms have succeeded in sur- 

 viving with very little change, and flourish to the present day. 



Evidently, then, evolution has been a very complex as well 

 as a very slow and uncertain process, and the intricacies of it 

 are not to be mastered by a slight, second-hand acquaintance 

 with zoological theory. 



