80 SECTIONAL ADDRESSES. 



common an ornament in Crustacea and other Arthropods and which 

 serve to stiffen the carapace. A very slight projection of any of these 

 tubercles further acts as a protection against such soft-bodied enemies 

 as jelly-fish. Longer out-growths enlarge the body of the trilobite in 

 such a way as to prevent its being easily swallowed. When, as is 

 often the case, the spines stretch over such organs as the eyes, their 

 protective function is obvious. This becomes still more clear when 

 we consider the relation of these spines to the body when rolled up, 

 for then they are seen to form an encircling or enveloping chevaux- 

 de-frise. But besides this, the spines in many cases serve as balancers ; 

 they throw the centre of gravity back from the weighty head, and 

 thus enable the creature to rise into a swimming posture. Further, 

 by their friction, they help to keep the animal suspended in still water 

 with a comparatively slight motion of its numerous oar-like limbs. 

 Regarded in this light, even the most extravagant spines lose their 

 mystery and appear as consequences of natural selection. A com- 

 parison of the curious Marrella in the pelagic or still-water fauna 

 of the Middle Cambrian Burgess shale with Acidaspis radiata of the 

 Calceola-beds certainly suggests that both of these forms were adapted 

 to a similar life in a similar envu-onment. 



The fact that many extreme developments are followed by the 

 extinction of the race is due to the difficulty that any specialised organism 

 or machine finds in adapting itself to new conditions. A highly 

 specialised creature is one adapted to quite peculiar circumstances : 

 very slight external change may put it out of harmony, especially if the 

 change be sudden. It is not necessary to imagine any decline of vital 

 force or exhaustion of potentiality. 



When people talk of certain creatures, living or extinct, as obviously 

 unadapted for the struggle of life, I am reminded of Sir Henry de la 

 Beche's drawing of a lecture on the human skull by Professor 

 Ichthyosaurus. 'You will at once perceive,' said the lecturer, 'that 

 the skull before us belonged to one of the lower orders of animals ; 

 the teeth are very insignificant, the power of the jaws trifling; and 

 altogether it seems wonderful how the creature could have procured 

 food.' 



What, then, is the meaning of ' momentum ' in evolution? Simply 

 this, that change, whatever its cause, must be a change of something 

 that already exists. The changes in evolving lineages are, as a rule, 

 orderly and continuous (to avoid argument the term may for the 

 moment include minute saltations). Environment changes slowly and 

 the response of the organism always lags behind it, taking small heed 

 of ephemeral variations." Suppose a change from shallow to deep water 



* The conception of lag in evolution is of some importance. On a hypothesis 

 of selection from fortuitous variations the lag must be considerable. If the 

 variations be determinate and in the direction of the environmental change, the 

 lag will be reduced ; but according as the determination departs from the 

 environmental change, the lag -will increase. If a change of environment acts on 

 the germ, inducing either greater variation or variation in harmony with the 

 change, there will "still be lag, but it will be less. On this hypothesis the lag 

 will depend on the mechanism through which the environment affects the germ. 

 If, with Oaborn, we imagine an action on the body, transmitted to the various 



