20 THE GENESIS OF SPECIES. [CHAP 



Again, the singular facts of " homology " are capable of 

 a similar explanation. " Homology " is the name applied 

 to the investigation of those profound resemblances which 

 have so often been found to underlie superficial differences 

 between animals of very different form and habit. Thus 

 man, the horse, the whale, and the bat, all have the pec- 

 toral limb, whether it be the arm, or fore-leg, or paddle, or 

 wing, formed on essentially the same type, though the num- 

 ber and proportion of parts may more or less differ. Again, 

 the butterfly and the shrimp, different as they are in ap- 

 pearance and mode of life, are yet constructed on the same 

 common plan, of which they constitute diverging manifesta- 

 tions. No a priori reason is conceivable why such simi- 

 larities should be necessary, but they are readily explicable 

 on the assumption of a genetic relationship and affinity be- 

 tween 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 under- 

 go 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 re- 

 semblances by means of which the adopted zoological and 

 botanical systems of classification have been possible finds 

 its solution in a similar manner, classification becoming the 

 expression of a genealogical relationship. Finally, by this 

 theory and as yet by this alone can any explanation be 

 given of that extraordinary phenomenon which is meta- 

 phorically 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 



