114 ANIMAL ECOLOGY 



applies to wild animals, and whether the analogy with man can 

 be followed up. If we go into the question carefully, it soon 

 becomes clear that there is an optimum density in numbers 

 for any one species at any one place and time. This optimum 

 number is not always the same and it is not always achieved, 

 but in a broad way there is a tendency for all animals to strike 

 some kind of mean between being too scarce and too abundant. 

 As examples we may take the domestic cat in two of its wilder 

 moments. Some years ago a schooner was run ashore on the 

 coast of Tristan da Cunha, a remote island in the Southern 

 Atlantic, and some of the ship rats were able to get ashore and 

 colonise the island. In a few months they bred and increased 

 excessively until they became quite a plague, even attacking 

 and eating rabbits on the island. The inhabitants accordingly 

 introduced some cats with the praiseworthy idea of extinguish- 

 ing the rats. But the rats were so very much more numerous 

 that they killed off the cats instead. ^^ In this case there were 

 not enough cats. They were overpowered by weight of 

 numbers. 



The other example is also about a small island, called Ber- 

 lenga Island, off the coast of Portugal. *" This place supports 

 a lighthouse and a Hghthouse-keeper, who was in the habit of 

 growing vegetables on the island, but was plagued by rabbits 

 which had been introduced at some time or other. He also 

 had the idea of introducing cats to cope with the situation, 

 which they did so effectively that they ultimately ate Up every 

 single rabbit on the island. Having succeeded in their object 

 the cats starved to death, since there were no other edible 

 animals on the island. In this case there were too many cats. 



21 . If we follow up further the analogy with human density 

 of population, it becomes clear that every animal tends to have 

 a certain suitable optimum which is determined mainly by the 

 habits and other characteristics of the species in question. 

 But these are continually changing during the course of evolu- 

 tion, and any such change is liable to cause a corresponding 

 alteration in the optimum density of numbers. For instance, 



* This incident was related to me by Mr. W. C. Tate, the well-known 

 authority on Portuguese birds, and is published here with his permission. 



