70 THE LESSON OF EVOLUTION 



its effects ; but in the earlier times, which we are 

 now considering, the problem was much simpler, and 

 it may not be impossible to solve it. 



The Graptolites were the first great group to suffer 

 extinction. Pelagic in habit, they could not have 

 suffered more than other pelagic animals from a 

 change in climate. Living on the minute organisms 

 which swarmed in the sea, and which they captured 

 with their tentacles, we can hardly suppose that they 

 succumbed to a want of food ; and we are thus led to 

 the conclusion that they must have formed food for 

 others. Who were these others? They must have 

 been either Medusae or pelagic Cephalopods, the 

 owners perhaps of some of the conodonts ; and of 

 the two I should be inclined to choose the latter, 

 but we know very little about them. 



With regard to the Trilobites, Professor Walcott 

 says that owing to their great differentiation the initial 

 vital energy of the group became impaired, and that 

 this was the cause of their extinction. With this I 

 cannot agree, for the reason already given, and must, 

 therefore, try to find some other and more efficient 

 cause at work. As the Trilobites lived on the bottom 

 of the ocean, where the temperature is uniform ; we 

 cannot invoke a change of climate as the cause of 

 extinction, and there does not appear to have been 

 any group of animals which could have been successful 

 competitors with them for their food, for we know 

 that they fed upon mud, which no doubt contained 

 numerous organic particles. So again we must have 

 recourse to predaceous foes. This reasoning is much 

 strengthened by the fact that in Ordovician and 

 Silurian times the Trilobites had learnt how to defend 



