io MANUAL OF ZOOLOGY 



of the changes by which, in the course of ages, it has been 

 evolved from an ancestral type. Evidence furnished by the 

 last-named circumstance is, of course, furnished by embry- 

 ology ■ the study of extinct animals constitutes a special 

 branch of morphology to which the name Palaeontology is 

 applied. 



The solid crust of the earth is composed of various kinds 

 of rocks divisible into two groups : ( i ) Igneous rocks, such 

 as granite and basalt, the structure of which is due to the 

 action of the internal heat of the globe, and which originate 

 below the surface and are not arranged in layers or strata ; 

 (2) Aqueous or sedimentary rocks, which arise by the disin- 

 tegration, at the surface of the earth, of pre-existing rocks, 

 the fragments or debris being carried off by streams and 

 rivers and deposited at the bottom of lakes or seas. Being 

 formed in this way by the deposition of successive layers or 

 strata, the sedimentary rocks have a stratified structure, the 

 lowest being in every case older than the more superficial 

 layers. The researches of geologists have shown that there 

 is a general order of succession of stratified rocks ; that they 

 may be divided into three great groups, each representing 

 an era of time of immense but unknown duration, and that 

 each group may be subdivided into more or fewer systems 

 of rocks, each representing a lesser period of time. 



Imbedded in these rocks are found the remains of various 

 extinct animals in the form of what are called fossils. In 

 the more recent rocks the resemblance of these to the hard 

 parts of existing animals is perfectly clear ; we find shells 

 hardly differing from those we pick up on the beach, bones 

 easily recognisable as those of mammals, birds, or fishes, 

 and so on. But in the older rocks the fossils are in many 

 cases so different in character from the animals existing at 

 the present day as to be referable to no existing order. We 



