C. — GEOLOGT. ( y 



and, casting off all restraint, rushes with accelerated velocity down 

 the easy slope to perdition. 



A melancholy picture! But is it true? The facts in the case of 

 the Cretaceous Polyzoa are not disputed, but they can be interpreted 

 as a reaction of the organism to the continued abundance of lime-salts 

 in the sea-water. If a race became choked off with lime, this perhaps 

 was because it could not keep pace with its environment. Instead of 

 ' irresistible momentum ' from within, we may speak of irresistible 

 pressure from without. Dr. Lang has told us (1919, Phil. Trans. 

 B. ccix.) ' that in their evolution the individual characters in a 

 lineage are largely independent of one another. ' It is this independ- 

 ence, manifested in differing trends and differing rates of change, that 

 originat'Cs genera and species. Did the evolution follow some inner 

 impulse, along lines ' predetermined and limited by innate causes,' 

 one would expect greater similarity, if not identity, of pattern and 

 of tempo. 



Many are the races which, seeking only ornament, have (say our 

 historians) perished like Tarpeia beneath the weight of a less welcome 

 gift: oysters, ammonites, hippurites, crinoids, and corals. But I see 

 no reason to suppose that these creatures were ill-adapted to their 

 environment — until the situation changed. This is but a special case 

 of increase in si^ie. In creatures of the land probably, and in creatures 

 of the water certainly (as exemplified by A. D. Mead's experiments 

 on the starfish, 1900, Amer. Natural, xxxiv. 17), size depends on 

 the amount of food, including all body- and skeleton-building con- 

 stituents. When food is plentiful larger animals have an advantage 

 over small. Thus by natural selection the race increases in size until 

 a balance is reached. Then a fall in the food-supply handicaps the 

 larger ci-eatures, which may become 'extinct. So simple an explana- 

 tion renders it quite unnecessary to regard size as in itself indicating 

 the old age of the race. 



Among the structures that have been most frequently assigned to 

 some blind growth-force are spines or horns, and when they assume 

 a grotesque form or disproportionate size they are dismissed as evidences 

 of senility. Let us take a case. 



The Trilobite family Lichadidae is represented in Ordovician and 

 Silurian rocks by species with no or few spines, but in the early 

 part of the Devonian, both in America and in Europe, various unrelated 

 groups in this family begin to grow similarly formed and situated 

 spines, at first short and straight, but soon becoming long curved horns, 

 until the climax is reached in such a troll-like goat-form as Ceratarges 

 armatus of the Calceola-Beds in the Eifel. 



Dr. J. M. Clarke (1913, Monogr. Serv. Geol. Brazil, i. p. 142) 

 is among those who have regarded this parallel development as a sign 

 of orthogenesis in the most mystical meaning of the term. Strange 

 though these little monsters may be, I cannot, in view of their con- 

 siderable abundance, believe that their specialisation was of no use 

 to them, and I am prepared to accept the following interpretation by 

 Dr. Rudolf Richter (1919, 1920). 



Such spines have their Ez'st origin in t-He tubercles which form sq 



