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look to changes of currents, climate, or other physical conditions, as the cause of these great mutations in the forms of life throughout the world, under the most different climates. We must, as Barrande has remarked, look to some special law. We shall see this more clearly when we treat of the present distribution of organic beings, and find how slight is the relation between the physical conditions of various countries,
and the nature of their inhabitants. |
This great fact of the parallel succession of the forms of life throughout the world, is explicable on the theory of natural selection. New species are formed by new varieties arising, which have
some advantage over older forms; and those
forms, which are already dominant, or have some advantage over the other forms in their own country, would naturally oftenest give rise to
new varieties or incipient species;
for these latter must be victorious in a still higher degree in order to be preserved and to survive.
We have distinct evidence on this head, in the plants which are dominant, that is, which are commonest
in their own homes, and are | in their own homes, and are 1859 1860 |
| and 1861 1866 1869 1872 |
| having produced 1859 1860 |
| compared with other plants within their own homes, having produced 1861 |
| compared with other less dominant plants, producing 1866 |
| producing 1869 1872 |
| domi- nant, 1859 1860 | | dominant, 1861 1866 1869 1872 |
| but 1859 1860 |
| or on the gradual acclimatisation of new species to the various climates through which they must pass, but 1861 |
| or on the gradual acclimatisation of new species to the various climates through which they have to pass, but 1866 |
| and on the gradual acclimatisation of new species to the various climates through which they might have to pass, but 1869 1872 |
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