| → though becoming under each grade 1861 1866 |
| by having become at each stage 1869 1872 |
|
| → within a confined 1859 1860 1861 1866 |
| generally within the same 1869 |
| within the same 1872 |
|
| → will generally be 1859 1860 1861 |
| generally are the 1866 |
| are the 1869 1872 |
|
| → under the most different climates; 1859 1860 1861 |
| and in the northern and southern temperate zones; 1866 1869 1872 |
|
|
fitted for simple conditions of life; it is likewise compatible with some forms having retrograded in organisation,
→though becoming under each grade
of descent better fitted for
and degraded habits of life. Lastly, the
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 a confined
the
and the extinct will
be
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
struck every traveller, namely, that on the same continent, under the most diverse conditions, under heat and cold, on mountain and lowland, on deserts and marshes, most of the inhabitants within each great class are plainly related; for they
→will generally be
descendants of the same progenitors and early colonists. On this same principle of former migration, combined in most cases with modification, we can understand, by the aid of the Glacial period, the identity of some few plants, and the close alliance of many others, on the most distant mountains,
→under the most different climates;
and likewise
|
