628 ZOOLOGY SECT. 



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

 Stegocephala, first appeared in the Devonian ; while all the earliest 

 vertebrate remains in the Cambrian and Silurian formations 

 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 structure. 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. 586). No 

 fewer than five parallel series of horselike Perissodactyles are 

 traceable, which developed and culminated separately, the cul- 

 minating 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. 



The first noteworthy attempt to solve the problem regarding the 

 nature of the forces by means of which evolution has taken place 

 was made, long before evolution was generally accepted among 



