have made it clear that such a scheme will be most likely to approach the truth, when its basis is laid in a thorough knowledge of the nature and extent of those variations which every chief modification of this type shows itself so peculiarly disposed to exhibit, and when in building it up the idea of natural affinity is accepted as expressing not only degree of mutual conformity, but actual relationship arising from community of descent more or less remote. For the endless gradational departures from any types which we may assume as fixed, and the occurrence of links of connexion between such as present the best marked differentiations, seem to me to point unmistakeably to this as the only escape from that difficulty of indefinite multiplication, which attends the application of the doctrine of distinct specific creations to a group in which scarcely any two individuals are alike.
The present aspect of this inquiry, in fact, may be not inappropriately compared with that of the oft-debated question as to the Races of Mankind. In the one case, as in the other, the direct evidence of descent affords cogent evidence as to the possible extent of modification within the limits of particular races; and when that evidence is brought into relation with analogous facts in regard to the yet greater variations of which we have direct evidence in the case of domesticated animals, it points to conclusions of higher generality, which physiologists find no difficulty in accepting. Now the modifications which any single type of Foraminifera must have undergone, to give origin to the whole series of diversified forms now presented by that group, are not greater in comparison with the modifications of which we have direct evidence, than are those which the advocate for the specific unity of the human races has no hesitation in assuming as the probable account of their present divergence.
This view of the case derives great force from the fact that there is strong reason to regard a large proportion of the existing Foraminifera as the direct lineal descendants of those of very ancient geological periods—a doctrine first advanced by Professor Ehrenberg in regard to a considerable number of Cretaceous forms, and since fully confirmed and extended as regards the Tertiary fauna by the admirable researches of Messrs. Rupert Jones and Parker, as well as by my own comparison of the recent and fossil types of Orbitolites, Orhiculina, Alveolina, Operculina, and Calcarina; and shown to be applicable also to the Secondary fauna, as far back as the Triassic system, by the remarkable results of the investigations of the same gentlemen in regard to a well-preserved sample of it. Following out, by laborious and extended comparison, the method of inquiry I have so much insisted on, they have found ample evidence that a like range has prevailed through the whole succession of geological periods to which their researches have extended. "Our own experience of the wide limits within which any specific group of the Foraminifera multiply their varietal forms, related by some peculiar conditions of growth and ornamentation, has led us to concur fully with those who regard nearly every species of Foraminifera as capable of adapting itself, with endless modifications of form and structure,
