392 RECAPITULATION. [CHAP. XV. 



of natural selection; for old forms are supplanted by new and 

 improved forms. Neither single species nor groups of species 

 reappear when the chain of ordinary generation is once broken. 

 The gradual diffusion of dominant forms, with the slow modifi- 

 cation of their descendants, causes the forms of life, after long 

 intervals of time, to appear as if they had changed simultaneously 

 throughout the world. The fact of the fossil remains of each 

 formation being in some degree intermediate in character between 

 the fossils in the formations above and below, is simply explained 

 by their intermediate position in the chain of descent. The grand 

 fact that all extinct beings can be classed with all recent beings, 

 naturally follows from the living and the extinct being the off- 

 spring of common parents. As species have generally diverged in 

 character during their long course of descent and modification, we 

 can understand why it is that the more ancient forms, or early 

 progenitors of each group, so often occupy a position in some 

 degree intermediate between existing groups. Recent 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 different 

 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 retrograded 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 migration 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 modification, 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 modification have been the same. 

 We see the full meaning of the wonderful fact, which has struck 

 every traveller, namely, that on the same continent, under the 

 most diverse conditions, under heat and cold, on mountain and 



