xxxii. 9 REPLACEMENT 775 



changed into some very different type of animal. Looking at any of 

 the evolutionary trees in this book, which represent, as it were, the 

 summary of the evidence about the populations, one notices at once 

 how frequently types common in earlier periods become extinct or are 

 replaced by their own descendants. Occasionally some line is recorded 

 as continuing over a long period, but the common picture is of change, 

 each type disappearing after a span of years. 



9. Successive replacement among aquatic vertebrates 



Does the examination of the sequences of types enable us to say 

 anything about the nature of these evolutionary changes? Can we 

 record any sense in which it represents a 'progress' or 'advance' ? One 

 striking feature that we have noticed is that often one type of organism 

 seems to replace another. There is always a difficulty in establishing 

 that this has occurred, because the fossil record does not leave us 

 sufficient information to show for certain that the two types occupied 

 identical 'niches'. However, if we take broad 'habitats', and particu- 

 larly those that change relatively little, such as the waters, we cannot 

 but be impressed with the succession of tenants that appears, each 

 replacing the one before (p. 237). Thus, among fish-like vertebrates 

 we can recognize ostracoderms, placoderms, crossopterygians, palaeo- 

 niscids, holosteans, and teleosteans, each almost completely replacing 

 the one before. 



Again, there has been an astonishing series of tetrapods returning 

 from the land to water, developing characters suitable for aquatic life 

 and then becoming extinct, apparently displaced by later migrants, 

 also returning from the land. To name only a few of these returners 

 we have among amphibians the phyllospondyls, lepospondyls, bran- 

 chiosaurs, and some urodeles; among reptiles the phytosaurs, croco- 

 diles, plesiosaurs, ichthyosaurs, mesosaurs, mosasaurs, aigialosaurs, 

 dolichosaurs, and snakes. Finally of the mammals there are the 

 basilosaurs, modern whales, seals, and sea-cows, as well as some less 

 completely aquatic types. 



However much we make allowance for the fact that the sea itself 

 may be changing, it is difficult not to find in these facts a suggestion 

 that the later types are replacing the earlier by their greater 'efficiency'. 

 These returned aquatics are especially interesting because each type 

 when it first re-enters the water seems to be not very well suited to 

 that medium — because of its shape for instance — and would therefore 

 be expected to be at a disadvantage in relation to the 'streamlined' 

 creatures that were already there. 



