emigrate or starve. Beyond a certain number the number of organisms living in one place cannot go.
But there are certain species of animals for whom the isolation, the division in individuals or pairs who live only for themselves, is the form of living which affords an advantage in the struggle for existence. Thus, for example, the cat species, which lie in wait for their booty, and take it with an unexpected spring. This method of acquiring their sustenance would be made more difficult, if not impossible, did they circulate in bigger herds. The first spring on the booty would drive all the game away for all the others. For wolves, which do not come unexpectedly on their prey, but worry it to death, the foregathering in herds affords an advantage; one hunts the game to the other, which blocks the way for it. The cat hunts most successfully alone. Again, there are animals who choose isolation because thus they are less conspicuous, and can most easily hide themselves, and soonest escape the foe. The traps set by men have, for example, had the effect that many animals which formerly lived in societies are now only to be found isolated, such as the beavers in Europe. That is the only way for them to remain unnoticed.
On the other hand, however, there are numerous animals which draw advantage from their social life. They are seldom beasts of prey. We have mentioned the wolf above. But even they only hunt in bands when food is scarce in winter; in summer, when it is easier to get, they live in pairs. The nature of the beast of prey is always inclined to fighting and violence, and, consequently, does not agree well with its equals. The herbivora are more peaceful from the very manner in which they obtain their food. That very fact in itself renders it easier for them to herd together, or to remain together, because they are more defencless; they will, however, through their greater numbers, need weapons in the struggle for life. The union of many weak forces to common action can produce a new and greater force. Then, through union, the greater strength of certain individuals is for the good
