for further advantageous elaboration to occur was by the enlargement of the competing unit. Various species of multicellular organisms might in time be supposed in turn to reach the limit of their powers. Competition would be at its maximum, smaller and smaller variations would be capable of producing serious results. In the species where these conditions prevail an enlargement of the unit is imminent if progress is to occur. It is no longer possible by increases of physical complexity and the apparently inevitable sequence is the appearance of gregariousness. The necessity and inevitableness of the change are shown by its scattered development in very widely separated regions (for example, in insects and in mammals) just as, we may suspect, multicellularity appeared.
Gregariousness seems frequently to be regarded as a somewhat superficial character, scarcely deserving, as it were, the name of an instinct, advantageous it is true, but not of fundamental importance or likely to be deeply ingrained in the inheritance of the species. This attitude may be due to the fact that among mammals at any rate the appearance of gregariousness has not been accompanied by any very gross physical changes which are obviously associated with it.3