I.] IXTROnUCTORY. 9 



the same type, though the number and proportion of parts 

 may more or less differ. Again, the butterfly and the 

 shrimp, different as they are in appearance and mode of 

 life, are yet constructed on one common plan, of which 

 they constitute diverging manifestations. No a priori 

 reason is conceivable why such similarities should be ne- 

 cessary, but they are readily explicable on the assumption 

 of a genetic relationship and affinity between the animals 

 in question, assuming, that is, that they are the modified 

 descendants of some ancient form — their common ancestor. 

 That remarkable series of changes which animals 

 undergo before they attain their adult condition, which 

 is called their process of development, and during which 

 they more or less closely resemble other animals during 

 the early stages of the same process, has also great light 

 thrown on it from the same source. The question as to 

 the singularly complex resemblances borne by every adult 

 animal and plant to a certain number of other animals 

 and plants — resemblances by means of which the adopted 

 zoological and botanical systems of classification have 

 been possible — finds its solution through the same hypo- 

 thesis, classification becoming the expression of a genea- 

 logical relationship. Finally, by this theory — and as yet 

 by this alone — can any explanation be given of that 

 extraordinary phenomenon which is metaphorically 

 termed mimicry. Mimicry is a close and striking, yet 

 superficial resemblance borne by some animal or plant to 

 some other, perhaps very different, animal or plant. The 

 "walking leaf" (an insect belonging to the grasshopper 

 and cricket order) is a well-known and conspicuous 

 instance of the assumption by an animal of the appear- 

 ance of a vegetable structure (see illustration on p. 40) ; 



