XX.] COMl'AEATIVE MORPHOLOGY. 249 



The only real explanation ever given of their existence is, 

 that they have been inherited from ancestors which had 

 the same or homologous organs in a functionally active 

 condition ; that the leg-bones of serpents, for instance, are Origin of 

 proof of their descent from animals that had legs, and the (jgscen/ 

 wing-bones of the apteryx are proof of its descent from a 

 bii'd that had wings. The legs and wings of the ancestral 

 forms have been lost as a result of that remarkable law 

 in virtue of which a disused organ diminishes not only 

 in strength but also in size.i 



Thus, though the law of adaptation is generally true. Exceptions 

 yet the existence of organs which are aborted, rudimen- adaptation 

 tary, and useless, shows that it is subject to exceptions, 

 and consequently is not a universal, ultimate, and aU- 

 explaining law. And though the law of homological 

 parallelism — or, to use the common and very appropriate and of 

 expression, the law of unity of type between organisms °™° °^' 

 that are externally unlike — is generally true ; as in the case 

 already mentioned of the hand of the man, the fore-foot of 

 the quadruped, the wing of the bat, and the paddle of the 

 whale ; yet the existence of such apparently capricious 

 deviations from the type as that of the number of the 

 cervical vertebrse in the sloth and the manati, shows that 

 unity of type is not an ultimate and universal law. 



On the view of all organisms whatever probaUy, and 

 certainly all organisms between which any unity of type Unity of 

 is discernible, being descended from the same ancestor, the ^^P^^j'^^^^ 

 law of unity of type is fully explained. It is simply a commu- 

 case of hereditary habit. The fact of externally unlike descent, 

 organs, as alluded to in the last paragraph, being formed 

 on what is fundamentally the same plan, is due to their 

 being inherited from the same ancestor. But what needs 

 further explanation, is the question how types have arisen Problem 

 and how they have been modified ; how, for instance, and and nSdi- 

 by what transitional stages, an original vitalized but fication 

 unorganized germ has been modified into the fish, and 

 the fish (which, as I shall show, is the original form of 



1 Their disappearance is, however, in gi-eat part due to natural selection, 

 of which I shall have to speak in a future chapter. 



