222 SOCIAL BEHAVIOUR 
in the trees. Learning, from frequent repetition of the fright, 
that the ground is a dangerous place, it has acquired the habit 
of building, roosting, and feeding on the high ¢rees ; and this 
habit is now operating for the preservation of this interesting 
bird, which a few years ago was almost extinct.” * Now, in 
this case, the young birds which followed the lead of those 
who, under experience, had acquired the habit, would stand a 
better chance of survival than those who, failing to do so, were 
caught napping on the ground. In further illustration, we 
may take the case of two species of rats found by Mr. C. M. 
Woodford on one of the Solomon Islands. These two species 
are regarded by Mr. Oldfield Thomas as slightly altered 
descendants of one parent species, with adaptations due to 
the fact that, of this original species, some have adopted a 
terrestrial, others an arboreal life. Thus J/us rex lives in 
trees, has broad footpads, and a long rasp-like, probably semi- 
prehensile tail ; while J/ws imperator lives on the ground, has 
smaller pads, and a short smooth tail. How far the different 
modes of behaviour in the two species may have been fostered 
by the influence of tradition we do not know; but it is not 
improbable that such an influence would be a co-operating 
factor in the process of segregation, and that in the course of 
time each form has been adapted to its special environment 
through the elimination of those individuals which were not 
in harmony with the conditions of their life. 
Such a case—admittedly hypothetical in the interpretation 
put upon the facts—may help us to see how the general 
instinctive follow-my-leader tendency might become specialized 
in certain essential lines of racial behaviour, and how, under 
natural selection, coincident variations in the line of traditional 
acts might become more and more definitely inherited as, at 
first, strong instinctive tendencies, and eventually more stereo- 
typed modes of instinctive behaviour. This, indeed, may 
have been the mode of origin of some of the social instincts. 
Reverting, however, to the stage where the general instinc- 
tive follow-my-leader tendency is only partly or incompletely 
* Proceedings of the Zoological Society, 1874, p. 184. 
