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 initiation is entirely withdrawn from observation, are 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 mknicry, or protection by 

 means of colouring and form. The first discoveries 

 on this subject were made by Bates, the well-known 

 K Naturalist on the Amazon;" the greater part were 

 subsequently added by Wallace. In South America, 

 the family of butterflies named Heliconida is extra- 

 13 



