when the development of their optic apparatus had not attained the degree of perfection which we now find in the stalk-eyed crabs and insects. They nevertheless agree not merely in their coarser conditions, but, as Max Schultze has demonstrated, even in their minutest microscopic details. If the idea of design as a principle of explanation is excluded in this case also, as will be shown below, and as is self-evident from our standpoint, and if simple heredity in the two series must be excluded. likewise, some other adequate solution must be sought.

The case of the converging species of sponges may throw a light, feeble though it be, upon the obscure processes of the organic laboratory. Let us here again recall that maxim of Goethe, which we have already cited: "The animal is formed by circumstances for circumstances." Perhaps this maxim may in future be brought into play, for it is actually a question of investigating how surrounding conditions, the agencies acting on the sensory apparatus, can exercise on simple matter such an influence, that the otherwise widely. differing descendants of the various possessors of this simple material or incomplete organs, have acquired a more complete organ, not only working in a similar manner, but of similar construction. Darwinism has never yet pretended to have explained everything; neither will it be wrecked on this point, but, on the contrary, will only have supplied fresh incitements to more profound researches, crowned by beautiful results.

Another example of approximation in divergent series is afforded by the eyes of the highest molluscs, the Cephalopods, as compared with those of the Vertebrata; in this instance, however, it does not go

It is only

beyond an analogy, though a striking one. the microscopic structure of the nerve membrane, which is extremely similar in both divisions, with the exception of the reversed sequence of the layers from inwards, outwards. The case, considered in the abstract, appears highly complicated, and without a prospect of solution; but it becomes marvellously simplified, as we have already hinted, if the question is thus generalized: In what manner are the still undifferentiated terminations of the nerves affected by the specific operation of the waves of light and sound, &c., so as to assume the form and construction of specific peripheral organs? It may be long before these relations. are fathomed; our only concern is to defend the theory from the reproach of inadequacy, by showing the scope for investigation according to our point of view.

When Darwin had brought to light the effects of natural selection in reproduction and derivation, and applied this principle to all the phenomena of the organic world, the systematic school was effectually subdued by that same doctrine of Descent, thus fortified and established, after which Lamarck had striven in vain. The systematic school classified organisms according to external and internal resemblances. Whence this greater or smaller accordance, whence the gradation and the heterogeneity, it knew not how to tell. It was thought that much was gained when the fundamental forms of types were spoken of, even though no account was given of the intrinsic nature of these types, floating like ideas above the phenomena. Now the type has become the family, and the systematizers have the plain task of restoring and combining the

pedigrees of the various groups of living beings. The knowledge of these pedigrees has now for the first time. a truly scientific purport, as compared with the old system of types; for the genealogical trees cannot be constructed without a knowledge of their growth, and of the causes which produced their branches, twigs, and shoots. Each family thus includes all the forms derived from one simple original form. The old systematic school was obliged to content itself with working out the classification of the individual types, and defining their limits, and then balancing the types against each other on general morphological and physiological principles, in order to estimate their relative value, all without any consciousness of the natural causes of these actual relations. The doctrine of Descent connects the original forms of the types afresh from the point of view of consanguinity, and descends deeper and deeper, down to the simplest organisms, and the beginning of life.

But before we attempt to come to an understanding as to the origin of life, one of the pillars of the doctrine of Descent, it seems appropriate to allude to the question whether natural selection, of which the means and effects will be more minutely elucidated in the following chapters, is capable of explaining all the modifications of organic beings, and whether selection must always be summoned to aid in the explanation of these transformations? In other words, whether the theory of selection answers all the requirements of the doctrine of Descent, or whether it is capable and in need of amendment? We may do this with the more impartiality, as the acute author of the book entitled "The Unconscious from the Stand

point of Physiology and the Theory of Descent (Das Unbewusste vom Standpunkt der Physiologie and Descendenztheorie),46 has again recently observed that the truth of the doctrine of Descent is independent of the bearings and adequacy of the Darwinian theory.

"This circumstance," he says, "is misunderstood by the majority of Darwin's opponents; when they adduce arguments for the inadequacy of natural selection in the struggle for life, they usually fancy they have adduced just as many arguments against the reliability of the theory of Descent. But the two have no direct connection with one another; for it might be possible that Darwin's theory of natural selection was absolutely false and unserviceable, and the doctrine of derivation true notwithstanding; that only the causal medium of the derivation of one species from another was different from that stated by Darwin. Similarly, it might be possible that, although the mediate causes of transition discovered by Darwin were partially effective,—on the other hand, transitional phenomena existed which could not as yet be explained by this hypothesis; that this therefore required either an auxiliary hypothesis supplementing that of Darwin, or even a co-ordinating principle of explanation, as little discovered now as was the Darwinian theory twenty years ago. Such imperfect knowledge of the causes operating in the transition of one form into the other, can prejudice the general truth of the doctrine of Descent as little as the absence of intermediate forms, or the uncertainty, still existing in many cases, of the derivation of any given form. If even in former times, when all knowledge of the causes by which transition is effected was still wanting,

the doctrine of Derivation seemed certain to the greatest minds, on philosophical and à priori grounds, there can be still less doubt as to the theory of Descent, now that Darwin and Wallace have plainly shown that indubitably the most important, if not the all-sufficing cause of transition is everywhere effective, and in many cases sufficient."

We wished to set forth these words of a talented philosopher for the benefit of those who are so unreasoning as to pour away the child with the bath-water, and fancy that they have slain the doctrine of Descent when they have been lucky enough to raise a few cavils against Darwin's theory of selection. Does the theory of selection fulfil every requirement? It accomplishes many and great things, but in some cases it seems to be inadequate, and in other cases it is not requisite, as the solution of the formation of species is found in other natural conditions.

Moritz Wagner, a decided adherent of Metamorphosis and an enthusiastic admirer of Darwin, endeavoured to establish a "law of migration," namely, that “the migration of organisms and the formation of colonies by them is the necessary condition of natural selection.” 47 In his opinion, new species arise only when smaller communities of individuals, in process of forming varieties, are geographically isolated, as in this manner only is intercrossing precluded with their stationary congeners, who do not participate in the transformation; and reversion and disappearance of characters as yet not fixed is thus avoided. That isolation often acts very favourably on the formation of species is a fact almost universally acknowledged and easily verified by insular fauna,