Page:Geological Evidences of the Antiquity of Man.djvu/436

416 species of a high class, a mammifer, for example, now passes, may be expected to present us with a picture of the stages through which, in the course of ages, that class of animals has successively passed in advancing from a lower to a higher grade. Hence the embryonic states exhibited one after the other by the human individual bear a certain amount of resemblance to those of the fish, reptile, and bird before assuming those of the highest division of the vertebrata.

Mr. Darwin, after making a laborious analysis of many floras, found that those genera which are represented by a large number of species contain a greater number of variable species, relatively speaking, than the smaller genera, or those less numerously represented. This fact he adduces in support of his opinion that varieties are incipient species, for he observes that the existence of the larger genera implies, in the period immediately preceding our own, that the manufacturing of species has been active, in which case we ought generally to find the same forces still in full activity, more especially as we have every reason to believe the process by which new species are produced is a slow one.

Dr. Hooker tells us that he was long disposed to doubt this result, as he was acquainted with so many variable small genera, but after examining Mr. Darwin's data, he was compelled to acquiesce in his generalisation.

It is one of those conclusions, to verify which requires the investigation of many thousands of species, and to which exceptions may easily be adduced, both in the animal and vegetable kingdoms, so that it will be long before we can expect it to be thoroughly tested, and, if true, fairly appreciated. Among the most striking exceptions will be some genera still large, but which are beginning to decrease, the conditions which were favourable to their former