182 THE STOEY OF THE EARTH AND MAS'. 



Secondly, these synthetic types often belonged to 

 the upper part of a lower group, or to the lower part 

 of an upper group. Hence in one point of view they 

 may be regarded as of high grade, in another as of low 

 grade, and they are often large in size or in vegetative 

 development.* From this law have arisen many con- 

 troversies about the grade and classification of the 

 Palaeozoic animals and plants. 



Thirdly, extinctions of species occur in .every great 

 oscillation of the continental areas, but some species 

 reappear after such oscillations, and the same genus 

 often recurs under new specific forms. Families and 

 orders, such as those of the Trilobites and Orthocera- 

 tites, appear to have a grand and gradual culmination 

 and decadence extending over several successive 

 periods, or even over the whole stretch of the Palaeo- 

 zoic time. Toward the close of the Palaeozoic, while 

 all the species disappear, some whole families and 

 orders are altogether dropped, and, being chiefly 

 synthetic groups, are replaced by more specialised 

 types, some of which, however, make small beginnings 

 alongside of the more general types which are passing 

 away. Our diagram (page 183) illustrates these points. 



Fourthly, the progress in animal life in the Palaeozoic 

 related chiefly to the lower or invertebrate tribes, and 



* It seems, indeed, as if the new synthetic forms intermediate 

 between great groups were often large in size, while the new 

 special types came Ji as small species. There are some remark- 

 able cases of this in the plant world ; though here we have such 

 examples as the pines and tree-ferns continuing almost un- 

 changed from an early Palaeozoic period until now. 



