``creatures'' 

or ``particles'', in the parlance of mathematicians who study ``contact processes''.

otherwise 

Of course, if we are fond of A and wish to think of it as a cute furry animal rather than some evil virus, we might want to refer to this boundary as a ``fitness'' rather than an ``epidemic'' threshold.

i 

Note that the connections between cells are asymmetric, i.e. i can ``infect'' j does not imply that j can ``infect'' i. A minor modification of this algorithm would render the connections symmetric, in which case the main qualitative features of the population dynamics would be similar to those presented here.

2 

Since the average total birth rates summed over all neighbors are held constant, this means that the birth rates along each individual link are quadrupled to compensate for the reduction in the number of neighbors.

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