3i6 ESSENTIALS OF BIOLOGY 



These express inferences are based only on morphological 

 resemblances and differences. 



At the present time all these types of living things exist, but 

 these categories are of particular significance : 



(i) The Thallophytes and, in particular, the Algae and Bacteria ; 



(2) The Flowering Plants ; 



(3) The Arthropods, particularly the Insects and Crustacea, 

 and 



(4) The Vertebrates, particularly the Teleostean Fishes and 

 Mammals, and we may say that these are the most ubiquitous and 

 dominant groups of living things on the earth. 



105. ON THE DEPLOYMENTS OF LIVING THINGS 

 Several times during the evolutionary career there has been 

 the sudden appearance of some type of organisms and then the 

 rapid divergence of a number of sub-types. Such deployments 

 have been : 



i. That of the Metazoan phyla in the Cambrian and Silurian 

 periods ; 



ii. The great deployment of the Pteridophytes — that is, the 

 arborescent Ferns, the arborescent Lycopods and Horsetails and 

 of the Pteridosperms during the Carboniferous period ; 



Hi. The spread over the earth, following the Cretaceous 

 period, of the flowering plants. 



Almost at once, as it appears, such groups of organisms have 

 evolved and become widely distributed, and there is no evidence, 

 in any case, of origins from other organic stocks. It may be the 

 case that the sudden deployment of the metazoan phyla at the 

 beginning of the Paleozoic periods is illusory and that there was 

 a long anterior process of divergent evolution that is concealed 

 by the great destruction of fossils during the period represented 

 by the unconformity that exists between the Paleozoic and Protero- 

 zoic strata. That unconformity may represent the destruction, 

 by erosion, of a very great thickness of strata which may have 

 contained transitional fossils. But there is no such extensive 

 unconformity between the Silurian and Devonian strata and yet 

 a similar deployment marks the first extensive land flora. Nor 

 is there any great unconformity between the Cretaceous and 

 Jurassic periods in which the records of the evolution of the 



