NATURAL SELECTION 71 



some animals have to encounter, among them the wild boar, 

 whom Sir Samuel Baker counted as the bravest of the brave. 

 Sportsmen and stoats keep down the number of rabbits, for 

 physical conditions with us are not often severe enough to 

 kill them. But with all species it is clear that disease must 

 account for a good many deaths. It is true that we seldom 

 find any sign of it among wild animals. The explanation, 

 however, is clear enough. Any ailment must speedily be 

 fatal, and individuals that have any congenital tendency to 

 disease cannot live long enough to leave any offspring. 

 We find a considerable crop of diseases making their appear- 

 ance among domestic animals. They are selected because of 

 some particular points which commend them to the fancy of 

 the breeder, not, like wild animals, because they combine very 

 great vigour with the other characters that are necessary to 

 the race. 



The struggle is not always one between individuals, but a The 

 herd of cattle, a rookery, a hive of bees, combine against all- 

 comers. Prince Kropotkin has, in some very interesting articles, groups 

 brought out clearly the fact that among animals there is an 

 enormous amount of mutual help. Nothing can be more true. 

 Wherever species are gregarious, they associate together for the 

 good of each and all. This does not prevent wrangling and 

 quarrelling, and gulls may often be seen on their feeding- 

 ground disputing over a sand worm or other choice morsel. 

 Still they keep together because they are sociable and friendly, 

 and very possibly, like rooks, mob a bird of prey on occasion. 

 But this friendliness never leads to any mitigation of the struggle 

 of the individual against disease. A sick bird is not fed or 

 tended in any way. 



Sometimes birds of one species help those of another, but 

 this is probably quite unintentional. The curlew (Numenius 

 arqnata) gives a signal which puts up numbers of other birds, 

 and the shore-shooter is angry and depressed. 



The combination is usually, but by no means always, for 

 defence. A hive of bees will make a combined attack 

 upon another and oust them, and several rats will band 



