EVOLUTION 551 



small filter-feeding organism of sluggish habits, which, were it 

 alive to-day, might be put in the same sub-phylum as Balanu- 

 glossits. The relationships of these creatures to invertebrates 

 are obscure ; the books which have been written to demonstrate 

 that the chordates are derived from annehds or arthropods 

 contain more invention than sense, and are chiefly interesting 

 as showing what the human mind can be made to believe when 

 there is no evidence. There are some similarities of embryology 

 between the chordates and the echinoderms, and also some points 

 of biochemistry in which they resemble each other and differ 

 from all other invertebrates. The only theory then which has 

 any support is that the chordate and echinoderm phyla once 

 had a common ancestor, but it is unlikely that it was much 

 above the coelenterate level. 



The next stage in chordate evolution was probably the acquisi- 

 tion of the postanal tail, and with it went a change in habits 

 to a more active life ; Branchiostoma and the ascidian tadpoles 

 are modern representatives of this level of organisation. Thus 

 far the chordates were marine, but at some time in the Ordovician 

 era they seem to have invaded the fresh waters and become ' fish ' 

 in the general sense of the term. The earliest fossils of this type 

 of which we know anything are the pteraspids, cephalaspids and 

 anaspids of the Silurian and Devonian periods ; they have all the 

 main vertebrate features (so far as these are capable of being 

 preserved in the rocks) except the jaw^s, and are a long way from 

 the lancelet. We must suppose that there is a long unknown 

 history, during which the vertebrate skeleton and other char- 

 acteristics were evolved. With the acquisition of jaws the Agnatha 

 developed into some sort of bony fish, and at this point the 

 vertebrates took two courses. As the Devonian freshwater lakes 

 dried up, some of the vertebrates returned to the sea, and became 

 the modern elasmobranchs and Actinopterygii ; the former are 

 still entirely marine, but some of the latter have for the second 

 time come into fresh water. The vertebrates which remained in 

 the lakes were the Choanichthyes, which had developed lungs 

 as an auxiliary method of breathing. The most successful of them 

 crept out into the air and became Amphibia, and the rest of these 

 abandoned the water altogether and became reptiles. From 

 the reptiles a much later bifurcation gave rise to the birds and 

 mammals. We shall now trace the development of the main 

 organ systems along these lines. 



