A VIEW IN PERSPECTIVE. 137 



is no doubt that there has been an increase in com- 

 plexity and diversity, and this is what is meant by 

 advance. At the same time, when we attempt to 

 follow this law into particulars it is by no means 

 always possible to do so. In the sub-kingdom Verte- 

 brata, there is no doubt of the facts. Beginning in 

 the Silurian (2) with the lowest form, there followed 

 in the Devonian (3) multitudes of fishes ; in the Car- 

 boniferous (4) came the next higher class, the Am- 

 phibia, to be followed in the next era by the 

 reptiles ; these by the birds in the Cretaceous and 

 the mammals in the Tertiary (9), and lastly by the 

 appearance of man in the Quaternary (10). In all 

 this we see a continuous advance. In the Articulata, 

 too, it is possible to trace the same advance. Ap- 

 pearing at first in the form of low generalized 

 trilobites and phyllocarides, the Crustacea developed 

 the higher orders of lobsters, shrimps, and crabs con- 

 siderably later. The insects seemed to appear in the 

 Silurian (2) as cockroaches, which belong to the 

 lowest order. In the other Paleozoic ages there 

 appeared indications of the Neuroptera, and Coleop- 

 tera, and Hemiptera, which are always recognized as 

 the lowest orders. It was only in the later periods 

 that the higher orders made their appearance. 



In the other groups of the animal kingdom, how- 

 ever, it is difficult to recognize any striking advance 

 in structure. It is questionable whether we can say 

 that the echinoderms of to-day are as a whole of a 

 higher type than those of the Silurian, and a like 

 question arises with respect to the Ccelentera, 

 Mollusca, and Brachiopoda. At the same time, the 



