[13] Ray Lankester, Encyclop. Brit., 9th ed., Vol. XIX, pp. 832-3.
[14] In most vertebrated animals this process of gastrulation has been more or less superseded by another, which is called delamination; but it scarcely seems necessary for our present purposes to describe the latter. For not only does it eventually lead to the same result as gastrulation—i. e. the converting of the ovum into a double-walled sac,—but there is good evidence among the lower Vertebrata of its being preceded by gastrulation; so that, even as to the higher Vertebrata, embryologists are pretty well agreed that delamination has been but a later development of, or possibly improvement upon, gastrulation.
[15] The most extreme of them is that which is mentioned in the last foot-note.
[16] For objections which may be brought against this and similar statements, see the [Appendix].
[17] For difficulties and objections, see Appendix.
[18] Heilprin, Geological Evidences of Evolution, pp. 73-4 (1888).
[19] Le Conte, loc. cit., pp. 236-7.
[20] I say “large areas” for the sake of argument; but the same correlation between distribution and affinity extends likewise to small areas where only small differences of affinity are concerned. Thus, for instance, speaking of smaller areas, Moritz Wagner says:—"The broader and more rapid the river, the higher and more regular the mountain-chain, the calmer and more extensive the sea, the more considerable, as a general rule, will be the taxonomic separation between the populations"; and he shows that, in correlation with such differences in the degrees of separation, are the degrees of diversification—i. e., the numbers of species, and even of varieties, which these topographical barriers determine.
[21] The only exception is in the case of the fish on each side of the Isthmus of Panama, where about 30 per cent, of the species are identical. But it is possible enough that at some previous time this narrow Isthmus may have been even narrower than at present, if not actually open. At all events, the fact that this partial exception occurs just where the land-barrier is so narrow, is more suggestive of migration than of independent creation.
[22] Origin of Species, pp. 353-4.