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ection with Lamarck's idea of transformative heredity； and applies the two in conjunction to the facts of histology。 He lays stress on the significance of functional adaptation； which I had described in 1866； under the head of cumulative adaptation； as the most important factor in evolution。 Pointing out its influence in the cell…life of the tissues； he puts 〃cellular selection〃 above 〃personal selection；〃 and shows how the finest conceivable adaptations in the structure of the tissue may be brought about quite mechanically； without preconceived plan。 This 〃mechanical teleology〃 is a valuable extension of Darwin's monistic principle of selection to the whole field of cellular physiology and histology； and is wholly destructive of dualistic vitalism。
The most important advance that evolution has made since Darwin and the most valuable amplification of his theory of selection is； in my opinion； the work of Richard Semon： 〃Die Mneme als erhaltendes Prinzip im Wechsel des organischen Geschehens〃 （Leipzig； 1904。）。 He offers a psychological explanation of the facts of heredity by reducing them to a process of （unconscious） memory。 The physiologist Ewald Hering had shown in 1870 that memory must be regarded as a general function of organic matter； and that we are quite unable to explain the chief vital phenomena； especially those of reproduction and inheritance； unless we admit this unconscious memory。 In my essay 〃Die Perigenesis der Plastidule〃 （Berlin； 1876。） I elaborated this far…reaching idea； and applied the physical principle of transmitted motion to the plastidules； or active molecules of plasm。 I concluded that 〃heredity is the memory of the plastidules； and variability their power of comprehension。〃 This 〃provisional attempt to give a mechanical explanation of the elementary processes of evolution〃 I afterwards extended by showing that sensitiveness is （as Carl Nageli； Ernst Mach； and Albrecht Rau express it） a general quality of matter。 This form of panpsychism finds its simplest expression in the 〃trinity of substance。〃
To the two fundamental attributes that Spinoza ascribed to substance Extension （matter as occupying space） and Cogitation （energy； force）we now add the third fundamental quality of Psychoma （sensitiveness； soul）。 I further elaborated this trinitarian conception of substance in the nineteenth chapter of my 〃Die Lebenswunder〃 （1904） （〃Wonders of Life〃； London； 1904。）； and it seems to me well calculated to afford a monistic solution of many of the antitheses of philosophy。
This important Mneme…theory of Semon and the luminous physiological experiments and observations associated with it not only throw considerable light on transformative inheritance； but provide a sound physiological foundation for the biogenetic law。 I had endeavoured to show in 1874； in the first chapter of my 〃Anthropogenie〃 （English translation； 〃The Evolution of Man〃； 2 volumes； London； 1879 and 1905。）； that this fundamental law of organic evolution holds good generally； and that there is everywhere a direct causal connection between ontogeny and phylogeny。 〃Phylogenesis is the mechanical cause of ontogenesis〃； in other words； 〃The evolution of the stem or race isin accordance with the laws of heredity and adaptationthe real cause of all the changes that appear； in a condensed form； in the development of the individual organism from the ovum； in either the embryo or the larva。〃
It is now fifty years since Charles Darwin pointed out； in the thirteenth chapter of his epoch…making 〃Origin of Species〃； the fundamental importance of embryology in connection with his theory of descent：
〃The leading facts in embryology； which are second to none in importance； are explained on the principle of variations in the many descendants from some one ancient progenitor； having appeared at a not very early period of life； and having been inherited at a corresponding period。〃 （〃Origin of Species〃 （6th edition）； page 396。）
He then shows that the striking resemblance of the embryos and larvae of closely related animals； which in the mature stage belong to widely different species and genera； can only be explained by their descent from a common progenitor。 Fritz Muller made a closer study of these important phenomena in the instructive instance of the Crustacean larva； as given in his able work 〃Fur Darwin〃 （1864）。 （English translation； 〃Facts and Arguments for Darwin〃； London； 1869。） I then； in 1872； extended the range so as to include all animals （with the exception of the unicellular Protozoa） and showed； by means of the theory of the Gastraea； that all multicellular； tissue…forming animalsall the Metazoadevelop in essentially the same way from the primary germ…layers。 I conceived the embryonic form； in which the whole structure consists of only two layers of cells； and is known as the gastrula； to be the ontogenetic recapitulation； maintained by tenacious heredity； of a primitive common progenitor of all the Metazoa； the Gastraea。 At a later date （1895） Monticelli discovered that this conjectural ancestral form is still preserved in certain primitive CoelenterataPemmatodiscus； Kunstleria； and the nearly…related Orthonectida。
The general application of the biogenetic law to all classes of animals and plants has been proved in my 〃Systematische Phylogenie〃。 （3 volumes； Berlin； 1894…96。） It has； however； been frequently challenged； both by botanists and zoologists； chiefly owing to the fact that many have failed to distinguish its two essential elements； palingenesis and cenogenesis。 As early as 1874 I had emphasised； in the first chapter of my 〃Evolution of Man〃； the importance of discriminating carefully between these two sets of phenomena：
〃In the evolutionary appreciation of the facts of embryology we must take particular care to distinguish sharply and clearly between the primary； palingenetic evolutionary processes and the secondary； cenogenetic processes。 The palingenetic phenomena； or embryonic RECAPITULATIONS； are due to heredity； to the transmission of characters from one generation to another。 They enable us to draw direct inferences in regard to corresponding structures in the development of the species （e。g。 the chorda or the branchial arches in all vertebrate embryos）。 The cenogenetic phenomena； on the other hand； or the embryonic VARIATIONS； cannot be traced to inheritance from a mature ancestor； but are due to the adaptation of the embryo or the larva to certain conditions of its individual development （e。g。 the amnion； the allantois； and the vitelline arteries in the embryos of the higher vertebrates）。 These cenogenetic phenomena are later additions； we must not infer from them that there were corresponding processes in the ancestral history； and hence they are apt to mislead。〃
The fundamental importance of these facts of comparative anatomy； atavism； and the rudimentary organs； was pointed out by Darwin in the first part of his classic work； 〃The Descent of Man and Selection in Relation to Sex〃 （1871）。 （〃Descent of Man〃 （Popular Edition）； page 927。） In the 〃General summary and conclusion〃 （chapter XXI。） he was able to say； with perfect justice： 〃He who is not content to look； like a savage； at the phenomena of nature as disconnected； cannot any longer believe that man is the work of a separate act of creation。 He will be forced to admit that the close resemblance of the embryo of man to that； for instance； of a dogthe construction of his skull； limbs； and whole frame on the same plan with that of other mammals； independently of the uses to which the parts may be putthe occasional reappearance of various structures； for instance of several muscles； which man does not normally possess； but which are common to the Quadrumanaand a crowd of analogous factsall point in the plainest manner to the conclusion that man is the co…descendant with other mammals of a common progenitor。〃
These few lines of Darwin's have a greater scientific value than hundreds of those so…called 〃anthropological treatises；〃 which give detailed descriptions of single organs； or mathematical tables with series of numbers and what are claimed to be 〃exact analyses；〃 but are devoid of synoptic conclusions and a philosophical spirit。
Charles Darwin is not generally recognised as a great anthropologist； nor does the school of modern anthropologists regard him as a leading authority。 In Germany； especially； the great majority of the members of the anthropological societies took up an attitude of hostility to him from the very beginning of the controversy in 1860。 〃The Descent of Man〃 was not merely rejected； but even the discussion of it was forbidden on the ground that it was 〃unscientific。〃
The centre of this inveterate hostility for thirty yearsespecially after 1877was Rudolph Virchow of Berlin； the leading investigator in pathological anatomy； who did so much for the reform of medicine by his establishment of cellular pathology in 1858。 As a prominent representative of 〃exact〃 or 〃descriptive〃 anthropology； and lacking a broad equipment in comparative anatomy and ontogeny； he was unable to accept the theory of descent。 In earlier years； and especially during his splendid period of activity at Wurzburg （1848…1856）； he h