"It should always be borne in mind what sort of intermediate forms must, on the theory [what theory?], have formerly existed. I have found it difficult when looking at any two species to avoid picturing to myself forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all its modified descendants. To give a simple illustration: the fantail and pouter pigeons are both descended from the rock pigeon. If we possessed all the intermediate varieties which have ever existed, we should have an extremely close series, between both and the rock pigeon; but we should have no varieties directly intermediate between the fantail and the pouter; none, for instance, combining a tail somewhat expanded with a crop somewhat enlarged, the characteristic features of these two breeds. These two breeds, moreover, have become so much modified that, if we had no historical or indirect evidence regarding their origin, it would not have been possible to have determined, from a mere comparison of their structure with that of the rock pigeon C. livia, whether they had descended from this species, or from some other allied form, as C. œnas.
"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]
To return, however, to Lamarck.
"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.
"It is only therefore the principal groups of animal and vegetable life which can be arranged in a vertical line of descent; species and even genera cannot always be so—for these contain beings whose organization has been dependent on the possession of such and such a special system of essential organs.
"Each great and separate group has its own system of essential organs, and it is these systems which can be seen to descend, within the limits of the group, from their most complex to their simplest form. But each organ, considered individually, does not descend by equally regular gradation; the gradations are less and less regular according as the organ is of less importance, and is more susceptible of modification by the conditions which surround it. Organs of small importance, and not essential to existence, are not always either perfected or degraded at an equal rate, so that in observing all the species of any class we find an organ in one species in the highest degree of perfection, while another organ, which in this same species is impoverished or very imperfect, is highly developed in another species of the same group."[257]
The facts maintained in the preceding paragraph are in great measure supported by Mr. Charles Darwin, who, however, assigns their cause to natural selection.
Mr. Darwin writes, "Ordinary specific characters are more variable than generic;" and again, a little lower down, "The points in which all the species of a genus resemble each other, and in which they differ from allied genera, are called generic characters; and these characters may be attributed to inheritance from a common progenitor, for it can rarely happen that natural selection will have modified several distinct species fitted to more or less widely different habits, in exactly the same manner; and as these so called generic characters have been inherited from before the period when the several species first branched off from their common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight degree, it is not probable that they should vary at the present day. On the other hand, the points in which species differ from other species of the same genus are called specific characters; and as these specific characters have varied and come to differ since the period when the species branched off from a common progenitor, it is probable that they should still often be in some degree variable, or at least more variable than those parts of the organization which have for a very long time remained constant."[258]