lOO 
The Irish NahcralUf. 
Aprils 
assumes that animals do not reach it. The puzzle then is, why 
are the animal inhabitants of the globe neither permanently 
increasing nor permanently decreasing their numbers ? Take 
the case of a bird which escapes the perils of winter scarcity 
by migrating to tropical countries, but which, during its long 
journey, is subjected to great fatigue, so that even in calm 
weather it falls an easy prey to the thousands of carnivorous 
galls that are ever on the watch for wearied migrants, while 
storms every now and then work wholesale destruction among 
its numbers, and many of those that survive the actual crisis 
are still so exhausted that they only reach land to die. These 
things happen. So far, we have fact to deal with, and not 
hypothesis. But unless they are followed by competition — in 
which case they have no effect on the ultimate number of 
surviving individuals — they leave us without a clue to the 
maintenance of a general average population. It becomes a 
mere chance whether the destruction is too great or too little 
for that purpose. The odds would be millions to one — in fact 
so great as to put the idea completely out of the question — 
against the chance of the average annual destruction by these 
catastrophes being exactly equal in amount to what is needed 
to prevent the species from increasing on the one hand, or 
from decreasing on the other. Unless, however, the loss is 
adjusted with this impossible nicety, the species will either 
decrease and become extinct, or it will increase until it 
reaches that very limit of subsistence which we are trying to 
keep out of court. We seem, therefore, to have been brought 
back against our will to the view on which animal life is at its 
high-water mark, and competition the ruling factor. 
The only escape, so far as I see, is in Professor Weismann's 
theorj^ that an animal subjected to greater loss than its rate of 
fertility can afford may save itself from extermination by 
developing either a higher rate of fertility or a longer average 
life, which would come to the same thing. Professor Weis- 
mann himself thinks that an increase in fertility would in many 
important cases be impossible, and that the difficulty can only 
be met by greater longevity. But it seems to me that a 
decreasing animal would have to modify its constitution very 
quickly, if it is to become sensibly longer-lived before it 
becomes extinct. We must remember that natural selection 
