ADAPTIVE COLORATION 227 
visible only, never extending to internal characters or to such 
as do not affect the external appearance.” 
These rules relate chiefly to the Batesian form of mimicry 
and need to be altered to apply to the Mullerian kind. 
The first criterion given by Wallace is evidently an essential 
one and it 1s sustained by the facts. It is also true that mimic 
and model occur usually at the same time of year; Marshall 
found many new instances of this in South Africa. In some 
cases of mimicry, strange to say, the precise model is unknown. 
Thus some Nymphalidz diverge from their relatives to mimic 
the Eupiceine, though no particular model has been found. 
In such instances, as Scudder suggests, the prototype may 
exist without having been found; may have become extinct; 
or the species may have arrived at a general resemblance to 
another group without having as yet acquired a likeness to 
any particular species of the group, the general likeness mean- 
while being profitable. 
The second condition named by Wallace is correct for 
Batesian but not for Mullerian mimicry. 
The fulfilment of the third condition is requisite for the 
success of Batesian mimicry. Bates noted that none of the 
pierid mimics were so abundant as their heliconid models. 
If they were, their protection would be less; and should the 
mimic exceed its model in numbers, the former would be more 
subject to attack than the latter. Sometimes, indeed, as 
Muller found, the mimic actually is more common than the 
model; in which event, the consequent extra destruction of the 
mimic would 
at least theoretically—reduce its numbers back 
to the point of protection. 
In Mullerian mimicry, however, the inevitable variation in 
abundance of two or more converging and protected species is 
far less disastrous; though when two species, equally distaste- 
ful, are involved, the rarer of the two has the advantage, as 
Fritz Muller has shown. His lucid explanation is essentially 
as follows: 
Suppose that the birds of a region have to destroy 1,200 
