THE PROGRESS OF LIFE 73 



Palaeozoic that we find any trace of land animals. 

 But when we pass to the second half, embracing the 

 time from the commencement of the Devonian to the 

 Pleistocene, we shall find that our attention will be 

 almost entirely directed to the land. 



Of course, during this time organic evolution went 

 on steadily in the ocean also ; but no new types 

 appeared there, except some air-breathing vertebrates 

 which were descended from land animals. The simple 

 and lowly forms of the Palaeozoic era, which came first 

 in the ocean, did not altogether die out ; for a type 

 once introduced almost always continued to exist. 

 But the various species and genera that is, the 

 particular shapes which represented the type 

 changed ; each, after living a certain time, became 

 extinct, and gave place to its successors. Usually the 

 simple types continued, living side by side with the 

 more complex or higher types, which successively came 

 into existence ; so that life in the ocean, as well as on 

 the land, became more and more varied as time went on. 



This evolution may be compared to the growth of a 

 hypothetical tree, in which the shapes of the leaves 

 slowly alter year after year. The leaves which die 

 out represent the extinct genera and species ; while 

 the branches represent the classes and orders which 

 continue to exist. Occasionally a bough may die also ; 

 but it is not a regular occurrence as it is with leaves. 



During the first half of the biological evolution, 

 when comparatively simple forms alone existed, the 

 struggle for existence was but slightly felt : species 

 existed for a long time, and spread far and wide over 

 the earth. But as the more complex and specialised 

 forms came into existence, during the second half of 



