PRESIDENT’S ADDRESS. 355 
forms of animals, which may either prey on the older inhabitants 
or be successful competitors for their food supply. Change in 
climate may, perhaps, sometimes account for the extermination of 
a group of terrestrial animals or plants, but it cannot have a wide 
influence on those groups living in the sea, which must have 
perished either from violence or from famine. The struggle for 
existence with other animals has, no doubt, generally been the 
most efficient cause of extinction, and with Pelagic animals it is 
probably the only cause. At the present day, and during all the 
latter half of the earth’s history, the struggle for existense has 
been so complicated that it is hardly possible to trace out its 
effects ; but in the earlier times, to which this address refers, 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 Medusz 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 themselves 
by rolling themselves up, a feat which the Cambrian Trilobites 
were not able to perform. Now the earliest powerful predaceous 
animals we know were the ground Cephalopods which first appear- 
ing in the Upper Cambrian rapidly increased in importance during 
the Ordovician and especially during the Silurian. The relative 
numbers of the Nautiloidea in these three periods being as 
1:9: 33, Inthe Cambrian and Ordovician periods the Trilo- 
bites had greatly increased, but in the Silurian they began to 
decline in numbers, and rapidly diminished during the Devonian 
