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256 Likenesses; or. Philosophical Anatomy 



sively 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 diverg- 

 ing descendants of some ancient common ancestor. 



A distinction had long been recognised, had been plainly 

 put forth by Professor Owen, between parts which resemble 

 each other in their function — analogous parts — and parts 

 which resemble each other in their position with regard 

 to neighbouring bodily structures — ho7)iologous 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 j^ct'^ts. Such 

 facts of 'homology' had been deemed deep mysteries. Ko 

 cb 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 suffi- 

 cient explanation seemed to have been supplied. A new 

 definition of homologous parts thus suggested itself. They 



