PALEONTOLOGY. 377 



creation towards the improvement of the type. Bronn recog- 

 nises the frequency of so-called " mixed forms " uniting in 

 themselves features which subsequently are distributed and 

 specialised in different related genera or families, but he takes 

 such forms to be incontrovertible evidence of the law of the 

 introduction of improved forms. 



As far back as 1849, L. Agassiz had distinguished progressive, 

 prophetic, synthetic, and embryonic types among fossil organ- 

 isms, and had attributed great importance to the prophetic 

 and embryonic types as fore-runners and signs of coming 

 changes in the organised relations. A similar conception was 

 afterwards conveyed by Richard Owen in his definition of 

 "plan forms" or "archetypes." 



Both Agassiz and Bronn gave particular attention to the 

 grades of differentiation and complexity, and to the systematic 

 rank of an animal type, and enunciated fundamental principles 

 of animal organisation. In 1854, Edward Forbes for the first 

 time in literature pointed out the significance of degeneration, 

 or retrogression of types, as shown in certain groups of animals. 



According to Bronn, two fundamental principles have guided 

 the whole succession of organisms from the oldest geological 

 period to the present time : first, an extensive and intensive 

 productive force continually increasing in power ; and second, 

 the nature and the variations of the external conditions. With 

 remarkable skill and ingenuity, Bronn elucidates the circum- 

 stances and events upon which the activity of the productive 

 force is dependent, as well as the varying conditions of the 

 atmosphere, the climate, the distribution of land and water, 

 the configuration of the successive land surfaces in the past 

 ages, and the influence of the varying conditions on the 

 animate creation. He infers from these considerations the 

 law of terripetal development. From a primaeval ocean rose 

 cliffs, islands, and continents ; the fauna of a universal ocean 

 was succeeded by the first settlement of land animals and 

 plants ; as the islands and continents increased in size, and 

 denudation altered their surfaces, new conditions of existence 

 were provided for terrestrial and fresh-water inhabitants, and 

 more complex correlations and differentiations of parts were 

 rendered possible. The faunas and floras of the older geo- 

 logical periods bore a tropical impress : the temperature cooled 

 very slowly, and as the conditions approached more nearly to 

 those of the present age, the strange-looking orders, families, 



