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The same assertion holds good in many other cases, as with Azalea and Camellia. And the striped varieties of these genera belong to the group of ever-sporting forms, and therefore will be considered later on. So it is with carnations and pinks, which occasionally vary by layering, and of which some kinds are so uncertain in character that they are called by floriculturists "catch-flowers."
It has been objected against Mr. Darwin's theory that if all species and genera have come to differ through the accumulation of minute but as a general rule fortuitous variations, there has not been time enough, so far as we are able to gather, for the evolution of all existing forms by so slow a process. On this subject I would again refer the reader to Mr.
LATITUDE. All goods and evils partake of latitude and altitude, and according to latitude have their genera, and according to altitude their degrees, 478. LAW. Divine law and rational are one law, 276. How the declaration, that no one can fulfil the law, is to be understood, 528.
In this collection there are extinct species of all the thirty-two genera, excepting four, of the terrestrial quadrupeds now inhabiting the provinces in which the caves occur; and the extinct species are much more numerous than those now living: there are fossil ant-eaters, armadillos, tapirs, peccaries, guanacos, opossums, and numerous South American gnawers and monkeys, and other animals.
In this respect they resemble the common rats; and, therefore, it is idle to talk of mere herbivorous genera of animals.
I have said a large genus, because as we saw in the second chapter, on an average more species vary in large genera than in small genera; and the varying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and most widely-diffused, vary more than do the rare and restricted species.
The manner in which such remains are occasionally carried by rivers into lakes, especially during floods, has been fully treated of in the "Principles of Geology." The remains of fish are occasionally useful in determining the fresh-water origin of strata. Other genera contain some fresh-water and some marine species, as Cottus, Mugil, and Anguilla, or eel.
Darwin called attention to the struggle for existence as a means of aggregating these slight modifications in a divergence sufficient to produce new species, genera, or families. His argument may be very briefly stated as follows: 1. There is in Nature a law of heredity; like begets like.
But, although the species are distinct, the genera are the same as those which characterise the Silurian rocks above, and none of the characteristic primordial or Cambrian forms, presently to be mentioned, are intermixed. The same may be said of a set of beds underlying the Arenig rocks at Ramsay Island and other places in the neighbourhood of St. David's.
But one of the most remarkable cases of the kind now claims our attention. The bird was probably evolved in the late Triassic or early Jurassic. It appears in abundance, divided into several genera, in the Chalk period.
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