"So with natural species, if we look to forms very distinct--for instance, to the horse and the tapir--we have no reason to suppose that links directly intermediate between them ever existed, but between each and an unknown common parent. The common parent will have had in its whole organization much general resemblance to the tapir and the horse; but in some points of structure it may have differed considerably from both, even perhaps more than they differ from each other. Hence in all such cases we should be unable to recognize the parent form of any two or more species, even if we closely compared the structure of the parent with that of its modified descendants, unless at the same time we had a nearly perfect chain of the intermediate links.
"By the theory of natural selection [surely this is a slip for "by the theory of descent with modification"] all living species have been connected with the parent species of each genus, by differences not greater than we see between the natural and domestic varieties of the same species at the present day; and their parent species, now generally extinct, have in their turn been similarly connected with more ancient forms, and so on backwards, always converging to the common ancestor of each great class; so that the number of intermediate and transitional links between all living and extinct species must have been inconceivably great. But assuredly if this theory [the theory of descent with modification or that of "natural selection"?] be true, such have lived upon the earth."[255]
"Though Nature," he continues, "in the course of long time has evolved all animals and plants in a true scale of progression, the steps of this scale can be perceived only in the principal groups of living forms; it cannot be perceived in species nor even in genera. The reason of this lies in the extreme diversity of the surroundings in which each different race of animals and plants has existed. These surroundings have often been out of harmony with the growing organization of the plants and animals themselves; this has led to anomalies, and, as it were, digressions, which the mere development of organization by itself could not have occasioned."[256] Or, in other words, to that divergency of type which is so well insisted on by Mr. Charles Darwin.