MIMICRY. 179 
to operate upon both, and in equal measure, we shall 
not obtain the same form of head. The development 
of the form of head must therefore be aided by a pre- 
existing tendency, and we must hence regard it as 
hereditary.” 
Haeckel likewise propounds a law of individual 
adaptation, which expresses the fact that, notwith- 
standing the closest kinship, individuals diverge in 
many ways. The cause of this difference, chiefly con- 
spicuous in the individuals of the same litter or brood, 
is, so far as it is not due to adaptation, inherent in 
the germs, and is transferred to them by fluctuations 
and differentiations in the conditions of nutrition in 
the parents, mostly beyond our ken. Other phenomena 
of indirect adaptation are exhibited in the occurrence 
of malformations, of which the causes must be looked 
for only in disturbances of nutrition in the parental 
organisms by which the progenitors themselves were 
not perceptibly affected. Here also belong the cases 
in which influences which have affected one sex only 
are manifested exclusively in posterity in the same 
sex. As may be seen, these processes, of which the 
initiation is entirely withdrawn from observation, are 
closely connected with the most obscure province of 
heredity. 
An extremely interesting and important form of 
adaptation is the so-called mimicry, or protection by 
means of colouring and form. The first discoveries 
on this subject were made by Bates, the well-known 
“Naturalist on the Amazon;” the greater part were 
subsequently added by Wallace. In South America, 
the family of butterflies named Heliconida is extra- 
