648 ZOOLOCiY skct. 



Mammals already existed, the separate evolution of the two classes 

 from lower forms would afford a sufficient explanation. Reptiles 

 extend as far back as the Permian. Amphibians, in the shape of 

 the Stegoccphala, first appeared in the Devonian ; while all the 

 earliest vertebrate remains in the Cambrian and Silurian forma- 

 tions appear to belong to the class of the Fishes. Within each of 

 these classes a progress is usually traceable from older, more 

 generalised types, along diverging lines, to the various specialised 

 forms existing at the present day. In some cases, however — notably 

 in the Amphibia, Reptilia and Aves — the orders first represented 

 have become entirely extinct, and have been succeeded by others 

 that made their appearance on the scene at a comparatively late 

 period. 



In certain cases among the Mammalia a number of closely 

 related stages have been discovered, showing, taken in their 

 chronological order, a gradually increasing specialisation of struc- 

 ture. One of the best known examples of this is that of the 

 Horse, to which attention is directed in the section on the 

 Mammalia (p. 606). No fewer than five parallel series of liorse- 

 like Perissodactyles are traceable, which developed and culminated 

 separately, the culminating member — viz., the genus Equus — of 

 one only of these series surviving to the present day. And there 

 are other families of Mammals, chiefly among the Ungulates (the 

 family of the Pigs and various families of Ruminants) in which 

 an equally complete history has been made out. 



The direct evidence of the evolution of the Invertebrates is, in 

 general, very imperfect. Some existing types of a comparatively 

 highly-organised character are to be recognised among the fossil 

 remains in the oldest formations— the Cambrian — in which definite 

 organic structures, if we except a few Radiolaria and Foraminifera, 

 are traceable. There is no trace of primitive fossil members of 

 the various invertebrate phyla, and the highly organised air- 

 breathing Arthropods are represented both by Scorpions and by 

 Insects as far back as the Silurian. Such remarkably complete 

 geological histories as have been traced in some of the Mammalia 

 are extremely rare in the Invertebrates. Such direct evidence, 

 however, as is obtainable, points to the probability of evolution, 

 and it may be inferred that the absence of primitive generalised 

 representatives of the invertebrate phyla is most probably due 

 to the imperfect character of the geological record. 



The Lamarckian Theory. — Supposing it to be regarded as 

 proved that the organic world has come to be as we find it by a 

 process of gradual evolution, we have next to inquire by what 

 agencies this process of development has been brought about. A 

 sketch of the history of thought on this subject will be given in 

 the section on the history of Zoology, and it will not be necessary 

 here to refer to more than the most important points. 



