256 LESSONS FKOM NATURE. [CHAP. VIII. 



seemed to so many persons to have this destructive effect 

 because, by and through it, similarities existing between the 

 parts of different animals came to be represented as ex 

 clusively due to blood-relationship between them. It was no 

 longer a wonder that the skulls of a monkey and a mud-fish 

 were essentially similar, if both these animals were the 

 diverging descendants of some ancient common ancestor. 

 A distinction had long been recognised, had been plainly 



put forth by Professor Owen, between parts which 

 andVomofo- resembled each other in their function analogous 



parts and parts which resembled each other in 

 their position with regard to neighbouring bodily structures 

 homologous parts. The wing of the humming-bird and the 

 wing of the humming-bird hawk-moth are analogous parts 

 they perform the same function as, in a less perfect degree, 

 does the parachute of the little lizard (Draco volans}. But 

 the bones which sustain that parachute and the ribs of the 

 humming-bird are homologous parts i.e., they have similar 

 relations of position to neighbouring bodily structures. The 

 parachute-bones and the wing-bones, on the contrary, are 

 analogous parts. Such facts of &quot; homology &quot; had been 

 deemed deep mysteries. No a priori reason could be given 

 why animals of the most different modes of life should have 

 been formed on similar patterns. The man, the horse, the 

 whale, and the bat, all have the pectoral limb whether arm, 

 fore-leg, paddle, or wing formed on one type, diverse as are 

 the uses to which these limbs are applied. Again, the 

 butterfly and the shrimp, different as they are in appearance 

 and mode of life, are constructed on one common plan, of 

 which they exhibit diverging manifestations. These facts 

 were recognised as facts, though no explanation of them 

 could be offered. But they became readily explicable on the 

 assumption of a blood-relationship, through actual generation 

 and descent from common ancestors. Here, then, appeared 

 to be the end of mystery with respect to homology a ready, 

 clear and sufficient explanation seemed to have been supplied. 

 A new definition of homologous parts thus suggested itself. 



