6o PRINCIPLES OF PALEONTOLOGY. 



those inhabiting our present oceans, we should in most in- 

 stances find differences so great as almost to place us in 

 another world. This divergence is the most marked in the 

 Palaeozoic forms of life, less so in those of the Mesozoic period, 

 and less still in the Tertiary period. Each successive formation 

 has therefore presented us with animals becoming gradually 

 more and more like those now in existence ; and though there 

 is an immense and striking difference between the Silurian 

 animals and those of to-day, this difference is greatly reduced 

 if we compare the Silurian fauna with the Devonian ; that 

 again with the Carboniferous ; and so on till we reach the 

 present. 



It follows from the above that the animals of any given 

 formation are more like those of the next formation below, 

 and of the next formation above, than they are to any others ; 

 and this fact of itself is an almost inexplicable one, unless we 

 believe that the animals of any given formation are, in part at 

 any rate, the lineal descendants of the animals of the preced- 

 ing formation, and the progenitors, also in part at least, of the 

 animals of the succeeding formation. In fact, the palaeon- 

 tologist is so commonly confronted with the phenomenon of 

 closely-allied forms of animal life succeeding one another in 

 point of time, that he is compelled to believe that such forms 

 have been developed from some common ancestral type by 

 some process of '^evolution.'' On the other hand, there are 

 many phenomena, such as the apparently sudden introduction 

 of new forms throughout all past time, and the common occur- 

 rence of wholly isolated types, which cannot be explained in 

 this way. Whilst it seems certain, therefore, that many of the 

 phenomena of the succession of animal life in past periods can 

 only be explained by some law of evolution, it seems at the 

 same time certain that there has always been some other 

 deeper and higher law at work, on the nature of which it 

 would be futile to speculate at present. 



Not only do we find that the animals of each successive 

 formation become gradually more and more like those now 

 existing upon the globe, as we pass from the older rocks into 

 the newer, but we also find that there has been a gradual pro- 

 gression and development in the types of animal life which 

 characterise the geological ages. If we take the earliest-known 

 and oldest examples of any given group of animals, it can 

 sometimes be shown that these primitive forms, though in 

 themselves highly organised, possessed certain characters such 

 as are now only seen in the young of their existing representa- 

 tives. In technical language, the early forms of life in some 



