MIMIC BY. 291 



complete."* Mr. Wallace travelled both in the western and eastern 

 tropics. The late Prof. Drummond records similar impressions 

 in Africa : — " On finding one of these insects, I have often cut a 

 small branch from an adjoining tree, and laid the two side by side 

 for comparison ; and when both are partly concealed by the hands 

 so as to show only the part of the insect's body which is free from 

 limbs, it is impossible to tell the one from the other. The very 

 joints of the legs in these forms are knobbed to represent nodes, 

 and the characteristic attitudes of the insects are all such as to 

 sustain the deception." f Every writer, in fact, who approaches 

 the subject of animal disguises, whether evolutionist or not, quotes 

 these insects as one of the strongest illustrations he can find, and 

 with ample warrant, for we may take these " Stick-insects " as 

 affording a typical instance of what is understood as protective 

 resemblance. The protection, however, cannot be complete, for 

 Wallace found the stomachs of certain Cuckoos full of them. J 



Now, it is a general postulate that this highly imitative and 

 protected form is due to the action of "natural selection," acting 

 on some incipient and original element of variation. As Mr. 

 Bates observed : — " Natural selection having, from the first, 

 favoured the species which offered variation in these parts, the 

 tendency to variability has become perpetuated by inheritance. ''§ 

 Or, as Mr. Darwin put it : — " Assuming that an insect originally 

 happened to resemble in some degree a dead twig or a decayed 

 leaf, and that it varied slightly in many ways, then all the varia- 

 tions which rendered the insect at all more like any such object, 

 and thus favoured its escape, would be preserved, whilst other 

 variations would be neglected and ultimately lost ; or, if they 

 rendered the insect at all less like the imitated object, they would 

 be eliminated." || We should therefore expect, if a perfect 

 geological record could unfold the ancestry of these insects, to 

 trace a gradual evolution of form for protective purposes under 



* ' Contributions to the Theory of Natural Selection,' p. 64. 



f ' Tropical Africa,' 4th edit. p. 173. 



I ' Tropical Nature,' p. 93. — In North America "Walking-sticks (Dia- 

 pheromera) are eaten by the Crow-Blackbird and two species of Cuckoos." — 

 S. D. Judd (American ' Naturalist,' vol. xxxiii. p. 462). 



§ " Descriptions of Fifty-two New Species of Phasmidce " (Trans. Linn. 

 Soc. vol. xxv. p. 323). 



|| ' Origin of Species,' 6th ed. p. 182. 



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