288 EECAPITULATIOff. [Chap. XV. 



progenitors of each group, so often occupy a position 

 in some degree intermediate between existing groups. 

 Eecent forms are generally looked upon as being, on the 

 -whole, higher in the scale of organisation than ancient 

 forms ; and they must be higher, in so far as the later 

 and more improved forms have conquered the older and 

 less improved forms in the struggle for life ; they have 

 also generally had their organs more specialised for dif- 

 ferent functions. This fact is perfectly compatible with 

 numerous beings still retaining simple and but little 

 improved structures, fitted for simple conditions of life ; 

 it is likewise compatible with some forms having retro- 

 graded in organisation, by having become at each stage 

 of descent better fitted for new and degraded habits of 

 life. Lastly, the wonderful law of the long endurance 

 of allied forms on the same continent, — of marsupials 

 in Australia, of edentata in America, and other such 

 cases, — is intelligible, for within the same country the 

 existing and the extinct will be closely allied by descent. 

 Looking to geographical distribution, if we admit that 

 there has been during the long course of ages much migra- 

 tion from one part of the world to another, owing to 

 former climatal and geographical changes and to the 

 many occasional and unknown means of dispersal, then 

 we can understand, on the theory of descent with modi- 

 fication, most of the great leading facts in Distribution. 

 We can see why there should be so striking a parallelism 

 in the distribution of organic beings throughout space, 

 and in their geological succession throughout time ; for 

 in both cases the beings have been connected by the 

 bond of ordinary generation, and the means of modifica- 

 tion have been the same. AYe see the full meaning of 

 the wonderful fact, which has struck every traveller 



