ANCIENT LIFE ON THE EAETH 35 



the latter half of the earth's history, the struggle for 

 existence has been so complicated that it is hardly 

 possible to trace out 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 any 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 Cephalo- 

 pods, 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 tempera- 

 ture 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, 



