GREAT STEPS IN ORGANIC EVOLUTION 821 



intricacy in structural detail and progressive advance to some new 

 level. Many animals which occupy a low position on the general 

 scale of being are extraordinarily complex. This is well illustrated 

 by the exquisite intricacy of Radiolarians among the Protozoa, 

 and again of many Sponges, such as the well-woven Venus Flower- 

 Basket (fitly named Euplectella), or the Glass Rope Sponge (Hyalo- 

 nema). So again among many of the Corals (such as the Blue 

 Helioporas). and similarly among Polyzoa, Echinoderms, and so 

 forth. Yet despite all their complexity in detail, none of these types 

 reach the higher level of main organisation represented by such 

 familiar animals as segmented worms. So among ancient fossils, 

 like the sponges, etc., aforesaid, there is often a much greater 

 intricacy of structure than in forms which must yet be ranked far 

 higher, when judged by the standards of essential differentiation of 

 structure and integration of function. Thus even successful types, 

 well adjusted to their environment, may edd}^ rather than progress 

 in their evolution, with the intrinsic variability of the organism 

 finding its expression in detailed intricacy, often of great beauty, 

 rather than in raising the type to a higher functional and structural 

 level. We may instance, for this eddying development, many extinct 

 Sponges, Corals, Crinoids, and Eamp-Shells. Or again after the 

 Ammonite type had reached its general acme, it thereafter specialised 

 into detailed complexity of the foldings of the suture-line, where the 

 successive septa are united to the surface of the shell. In these 

 ornamented sutures, of great value in classification, we can see little 

 or no functional value. The same detailed complexity is well illus- 

 trated by the fantastic individuality of armature indulged in by not 

 a few Cretaceous Dinosaurs. Or again, while we can see substantial 

 use in the complex folding of the elephant's molar teeth, surely the 

 like in certain of the Carboniferous amphibians, appropriately 

 termed Labyrinth odonts, shows an elaboration which suggests 

 a veritable superfluity of eddying. 



Another large fact which a survey of the geological succession at 

 once discloses is the frequent extinction of highly developed types. 

 We can readily understand the elimination of the earlier stages in 

 any continuing evolutionar\^ series, but it is surprising to find the 

 extinction of entire races, so thorough that their lineage came to its 

 end. From the generalised crocodilians of the Triassic and Jurassic 

 Periods there arose a number of more specialised genera, which have 

 long since become extinct ; yet the race of Crocodilians is still well- 

 nigh world-wide — witness the Crocodiles of Africa, Tropical America, 

 West Indies, India, Malay, and North Australia; the Alligators of 

 the Southern States and of China ; the Caimans of Central America 

 and South America, and the long-snouted Gavial of India. 



This is an instance of the obliteration of the earlier stages in an 

 evolution series, with survival of well-adapted ones; and the same 



