EVOLUTION, VARIATION AND HEREDITY 457 



etc., etc. In each case they can be traced back to more primitive, 

 less specialised forms. 



These cases are, however, specific instances, but when 

 we turn to the general evidence from palaeontology we shall find that 

 it is just as strong. Let us glance for a few moments at the sequence 

 in which the various classes of the Vertebrata appear ; but in order 

 to do so we must first fill in the subdivisions of the other two geo- 

 logical eras (vide diagram). Throughout the whole of the Archaean 

 or Proterozoic and major part of the Cambrian we find no trace of 

 any animal with a vertebrate structure. They first appear possibly 

 in late Cambrian, but certainly in the Silurian, and these are the fish. 

 In the next period, the Devonian, we get more fish and the earliest, 

 most primitive Amphibia, and soon after in the Carboniferous other 

 and more highly developed Amphibia and very primitive amphibia- 

 like reptiles. The Permian period saw the origin of a number of 

 new and more developed Reptilia, which however increased greatly 

 in variety and organisation, so that they were the dominant forms 

 during the greater part of the Mesozoic Era, which is in consequence 

 termed the " Age of Reptiles." One line of reptilian development 

 led in the upper Triassic to the primitive Mammalia. These are 

 undoubted mammals, and the group has gradually gone on, and in 

 the Caenozoic Era it replaced the Reptilia as the dominant group. 

 The culminating and most recent addition to the Mammals is man 

 himself, who first appears in the Pliocene period. With the spread 

 of our knowledge of fossil forms that has taken place in the last twenty 

 or so years, it has been increasingly difficult to separate off the 

 Reptilia from the Amphibia on the one hand and the Mammalian 

 on the other. 



The birds appear as a highly specialised offshoot from the reptiles 

 in the Jurassic, which also furnishes us in its early layers with that 

 remarkable mixture of bird and reptile Archaeopteryx. 



This short account suffices to show the kind of evidence that 

 we derive from Anatomical, Embryological, Geographical and 

 Palaeontological sources. The amount of evidence that has now 

 been amassed is colossal, and it is being added to year by year. It 

 all points most clearly to the fact that living matter was at first 

 simple, but through countless ages it has gradually become more and 

 more specialised and produced an endless variety of forms. This 

 long-continued and continuous process of change or Evolution has 

 resulted in the production of all the many animal and plant forms at 

 present inhabiting the earth. 



Darwin. The most masterly contribution to the subject of 

 Evolution was not made until fifty years after the appearance of 

 Lamarck's book, when Charles Darwin published his " Origin of 



