CH. xii.] ADJUSTMENT, DIRECT AND INDIRECT. 63 



and has a purely physiological significance. That is to say, 

 the resemblance of the pterodactyl to carinate birds is a 

 secondary adaptive resemblance, like the less marked re 

 semblance of bats to birds, or like the resemblance of a 

 porpoise to a fish. And this view, which seems to be Prof. 

 Huxley s, is rendered probable by the fact that in wing- 

 structure the pterodactyl differs from birds in much the same 

 way that a bat does. 



We are now extricated from our imbroglio with regard to 

 classification, but we are still left confronted with the diffi 

 culty of supposing that the natural selection of casual varia 

 tions can so often have resulted in producing whole orders of 

 closely-resembling animals from distinct ancestral orders. 

 Other facts, brought up by Mr. Mivart, still further increase 

 the apparent difficulty. The most important of all these 

 relate to the development of the higher organs of sense in the 

 three sub-kingdoms of annulosa, rnollusks, and vertebrates. 

 Coincidences between the members of any one of these 

 sub-kingdoms and the members of the others, are not 

 to be attributed to community of origin. No naturalist 

 supposes that an annulose animal, or a true mollusk, has 

 ever been developed into a vertebrate. And while the mol- 

 lusks and vertebrates appear to have diverged from a mol- 

 luscoid ancestor akin to the still-living ascidians, the annulose 

 sub-kingdom has a totally different pedigree. To discover 

 any likeness between the two great groups, we must follow 

 them back to those remotest ancestors who possessed hardly 

 any distinctively animal characteristics. Bearing all this 

 in mind, it is a striking fact that the eye of the cuttle- 1 

 fish, which is the highest of mollusks, appears to be con- I 

 structed like the eyes of vertebrates. It apparently contains 

 not only a similar retina, but also a lens, the choroid and 

 sclerotic tunics, and the vitreous and aqueous humours. 

 Now this coincidence cannot be due to community of in 

 heritance, for the vertebrate and molluscous sub-kingdoms 



