ics of the biogenetic process, we must descend into the deep obscurity of plastid-life, and search for its true efficient cause in the motion of organic molecules (Plastidule-Bewegung)

"In fine, this question remains to be answered, Are we in a position, by the aid of comparison with analogous phenomena of motion, to form a satisfactory provisional hypothesis regarding the true nature of the plastidule motions which are hidden from our direct observation? Our hypothesis of perigenesis is an attempt to answer this question in the affirmative.

"As we review, from the highest and most comprehensive point of view, the sum of the phenomena of organic development, the most general result of our survey is the conclusion that the biogenetic process is a periodic motion, which we can best picture to ourselves as a wave motion. Adhering at first to facts which are beyond dispute, and which admit of direct observation, we may commence with our own ancestry: either confining ourselves to the so-called historic period, in which we can pass from man to man by direct proof; or else following, by the methods of anthropogeny, our ancestry still farther back, through the vertebrates to amphioxus, and through the group of invertebrates to the gastræa, and at last to the amoeba and the moner. In either case the course of development (entwickelungsbewegung) of our series of ancestors can be most simply represented by a wave-line, in which the individual life of each organism answers to a single wave.

"If now we enlarge our field of view to embrace not simply our own direct ancestry but the whole of our blood-relations, we can make clear by a genealogical tree their relationship to each other. As the history of the

evolution of each person is represented by a wave-line, the entire tree will have the form of a branched wavemotion, a ramified undulation. . .

"A natural system of classification is nothing but a genealogical tree of allied species of organisms, and each branch and twig of the tree corresponds to a greater or smaller group of descendants from a common ancestral form. This community of descent unites all the forms of a class, an order, and so on. Since each class is divided into various orders, each order into several families, each family again into various genera, each genus into a number of species and varieties, there is a similar branching in the wave-motion which is carried from the common ancestral form to the entire group of its descendants; and each undulating branch implants in the same way its individual motion on its various descendants.

"Now the fundamental law of embryology teaches us that this history of the philogenetic evolution of organisms is mirrored in miniature in the ontogenetic development of each individual. Here the single waves answer to the life of the constituent plastids (cytodes and cells). The cytula, or the first segmentation cell which originates from the fertilized egg, and out of which the many-celled organism is developed, bears the same relation to the various cell-generations which originate from it by division, and which give rise later by specialization of function to the various tissues, that the stemform of a class or order bears to the various families, genera and species which diverge from it, and which have been differently evolved through adaptation to diversified conditions of existence.

"The ontogenetic cell-tree' of the former has exactly the same form as the philogenetic species-tree' of the

latter. The developing impulse which in the one case is transferred from the ancestral species to the entire group of species, and in the other case from the ancestral cell to the entire group of cells, assumes in both cases the same form of a branching wave-motion. Any one who accepts the fundamental law of development will find it only natural that the microcosm of the ontogenetic cell-tree' should be a diminution, and to some degree distorted reflection of the phylogenetic 'speciestree.'

"As we can only explain and render intelligible a complicated and obscure phenomenon by dividing it into its separate elements, and by the exact analysis of these parts, so it is necessary to penetrate to the ultimate elementary facts of our mechanical theory of development.

"Now the biogenetic process as a whole is the highly compound resultant of the developmental history of all species of organisms. These consist again of the life histories of the individuals, just as the latter are again made up of that of the constituent plastids.

"The development of each plastid, however, is in its turn only the product of the active movements of its constituent plastidules. Now we have seen that the developmental impulse of the branches and classes, the orders and families, the genera and species, the individuals and plastids, always and everywhere has for its fundimental characteristics the branched wave-motion. Accordingly the molecular plastidule-motion, which lies at the bottom of all the phenomena of life, can have no other form. We must conclude that this ultimate cause of all the phenomena of life, that the invisible activity of the organic molecules is a branched wave-motion. This true and ultimate causa efficiens of the biogenetic

process I propose to designate by a single word-Perigenesis, the periodic wave-generation of the organic molecules or plastidules.

"This mechanical hypothesis is a true explanation of the process of organic development.

"The designation of this branched wave-motion of the plastidule by the word perigenesis or wave generation serves to emphasize the distinctive characteristic which separates this branched motion from all similar periodic phenomena. This peculiarity depends upon the reproductive power of the plastidule, and this again is brought about by its peculiar atomic composition. This power of reproduction which alone renders possible the multiplication of the plastids is, however, the equivalent of the memory (Gedächtness) of the plastidule.

"This brings us to Ewald Hering's ably established view that unconscious memory is the most important characteristic of organized matter, or more properly of the organizing plastidules. Memory is the chief factor in the process of development of organisms. Through the memory of the plastidules the plasson has the power to carry over from generation to generation by inheritance, in continuous periodic motion, its characteristic peculiarities, and to add to these the new experiences which the plastidules have acquired through adaptation in the course of their evolution.

"I have shown that each organic form is the necessary product of two mechanical factors-an inner factor, heredity, and an outer factor, variability, or a power of adaptation.

"By the hypothesis of perigenesis we are able to more sharply define these two fundamental laws of the modification of organisms, for heredity is the memory of the plastidules: variability their power of perception (Die

Erblichkeit ist das Gedächtniss der Plastidule, die Variabilität is die Fassungskraft der Plastidule). The one brings about the constancy and the other the diversity of organic forms. In the very simple and persistent forms of life the plastidules have, so to speak, learned nothing and forgotten nothing. In highly perfected and variable organisms the plastidules have both learned. and forgotten much."

This somewhat long quotation contains a thorough and exhaustive statement of the perigenesis hypothesis, and it is therefore interesting to notice that its only real claim to recognition as a true explanation of the phenomena of heredity is based upon or at least demands the acceptance of some form of the evolution hypothesis.

However great may be the importance of the analogy between the gradual evolution of the species by the specialization of the constituent individuals, and the development of the individual by the specialization of cells, and plastidules, we have already pointed out that it is in no sense an explanation of the latter, since the real cause of the evolution of the species, the selection of congenital variations, is absent.

The only part of Haeckel's hypothesis of perigenesis which has any claim to be considered an explanation of the reproductive power of animals, is the statement that heredity is memory, and variability the acquisition of new experiences. Stated by itself, without explanation, this may seem to those who are unfamiliar with the subject very much like nonsense, for the profound truth upon which it rests is not at all obvious at first sight.

Herbert Spencer has, in his masterly discussion of the nature and distinctive characteristic of life, given us, as the sum and substance of his analysis, the statement