Oct. 4, 1877] 



NATURE 



493 



building material for its execution. An immense literature and 

 an admirable perfection of the methods of investigation now give 

 the most brilliant proof of the astonishing progress of the empi- 

 rical science of nature during that period. But of course the 

 immeasurable widening of the field of empirical ob;ervation, 

 and the special division of labour caused by this, often led to an 

 unfortunate dispersion of powers ; the hijher object of the 

 recognition of general laws was often entirely forgotten in the 

 nearer interest in the observation of details. 



Thus it could happen tliat while this strictly empirical inves- 

 tigat'on of nature was flourishing at its highest in the years 

 1S30 to 1S59, during thirty years, the two principal branches 

 of real natural hislory started from totally different bases. In 

 the history of the development of the eartli the conviction gained 

 ground more and more since 1S30, the year of the publication 

 of " Lyell's Principles of Geology," that our planet had neither 

 been formed by a supernatural act' of creation, nor had pissed 

 through a series of total revolutions of mystical origin, but thaf, 

 on the contrary, a gradual and uninterrupted development had 

 caused its natural formation step by step. On the other hand, 

 in the history of development of the living inhabitants of tlie 

 earth the old irrational myth remained in full force, according to 

 which every single species of animals and plants, like man him- 

 self, had been created independently of one another, and that a 

 series of such creations had followed eacli other without any 

 genetic connection. The glaring contradiction of the two doc- 

 trines, of the natural development-theory of geologists, and of 

 the supernatural creation myth of biologists, was only decided in 

 favour of the former by D.irwin in 1S59. Since then we recog- 

 nise clearly that the formation and change of forms of the living 

 inhabitants of our globe follow the same great eternal laws of 

 mechanical development as the earth itself and the whole world- 

 system. 



We need not to-day, as we were obliged to do fourteen years 

 ago at the meeting of naturalists at Stettin, cite the reasons and 

 proofs for Darwin's new theory of development. The recog- 

 nition of its truth has since made its way in the most satisfactory 

 manner. In that domain of natural investigation to which my 

 own labours belong, in the wide field of the science of organic 

 forms or tiwyphology, it is already recognised everywhere as the 

 moit important basis. Comparative anatomy and the history of 

 germs, systematic zoology and botany cannot to-day do without 

 the theory of descent. Because only by its light the mysterious 

 relations of the numberless organic forms amongst each other 

 can be really explained, i.e., reduced to mechanical causes. 

 Their similarity results as the natural consequence of inhcritamc 

 from common parental forms, their variation as the necessary 

 effect of adiittatiou to different conditions of life. Only by the 

 theory of descent can the facts of palaeontology, of chorology, and 

 of o-'cology, be explained in a way as simple as it is natural ; 

 only by this theory we understand the existence of the remark- 

 able rudimentary organs, of the eyes which do not see, the wings 

 which do not flv, the muscles which do not move — nothing but 

 useless parts ot the body, which refute in the most emphatic 

 manner the old-fashioned tdeoli>s;y ; because they prove in the 

 clearest manner that the utility in the structure of organic 

 forms is neither general nor perfect ; that it is not the result of 

 a plan of creation worked with an object in view, but necessarily 

 caused by the accidental coincidence of mechanical causes. 



Who, in the face of these overwhelming facts, still asks to- 

 day fur proofs of the theory of descent, proves by that only his 

 own want of knowledge or reason. But it is utterly wrong 

 to demand exact or indeed experimental proofs. This demand, 

 w^Tich is so often heard, results from the widely-spread error 

 that all natural science must be exact ; all the other sciences 

 are often confronted with this, under the name of "spni- 

 tual or pure sciences " {Gcislcswissenschaflcii). Now in truth, 

 only the smaller part of natural science is exact, viz., that 

 part which can be proved mathematically ; astronomy before 

 all others, and higher mechanics in general ; after these the 

 greatest part of what remains of physics and chemistry, also a 

 good part of physiology, but only a very small part of morpho- 

 logy. In this latter biological domain the phenomena are far 

 too complicated and variable to allow of our applying the 

 mathematical method at all. If indeed the demand tor a foun- 

 dation, whicli shall be as exact as possible, and mathematical if 

 possible, stands good in principle for all sciences, it is yet quite 

 impossible to carry this tlirough in by far the greater part of the 

 biological disciplines. Heie, on the contrary, the historical and 

 historico-philosophical method takes the place of the exact, 

 mathematical, and physical one. 



This applies to morphology before all others, because the 

 scientific understanding of organic forms we obtain solely 

 through the history of their development. The great progress of 

 our time in this domain consists in our conceiving the meaning 

 and object of the history of development in an infinitely wider 

 sense than has been done before Darwin. Up to his time it 

 meant only the history of the formation of the organic indi- 

 vidual form, which to-day we call history of the germ, or 

 ontogeny. 



If the botanist followed the formation of the plant from the 

 seed, the zoologist that of the animal from the ovum, they con- 

 sidered their morphological task accomplished by the perfect 

 observation of the history of these germs. The greatest men in 

 the domain of the history of evolution, Wolff, Baer, Kemack, 

 Schleiden, and the whole school of embryologists formed by 

 I hem, understood by it, until a short time ago, the individual 

 ontogeny exclusively. It is quite different to-diy, when the 

 mysteries of the wonderful history of germs confront us no longer 

 as unintelligible riddles, but have clearly revealed their deep 

 significance ; because according to the laws of inheritance, the 

 changes of form which the germ passes through in the shoitest 

 time, under our eyes, are a compressed and abbreviated repeti- 

 tion of the corresponding changes of form, which the ancestors 

 of the organism in question have passed through in the course 

 of many millions of years. If to-day we place a hen's egg into 

 the breeding machine, and if twenty-one days later we see a little 

 chicken creep from it, we no lontjer remain in mut'^ astonish- 

 ment at the wonderful changes which lead from the simple cell 

 in the egg to the two-leaved gastrula, from this to the worm- 

 shaped and skull-less germ and thence to further germ-forms, 

 which on the whole show the organisation of a fish, an amphibian, 

 a reptile, and only lastly that of a bird. On the contrary, we draw 

 conclusions from this lega'dmg the c irresponding series of forms 

 of the ancestors, wliich have led from the unicellular aniceba to 

 the parental form of the gistrsea, and futhtr <m through the 

 classes of worms, acephala, fishes, ampin i'ia, reptiles, down to 

 birds. The series of gcrm-fornis of ihe chicken thus gives us a 

 ketch of the series of its real ancestors. 



Our biogenetic fundamental law gives the immediate causal 

 connection winch thus exists between the ontogeny of any 

 organic individual foim and the history of the forms of its 

 ancestors in the following short phrase : — The history 0/ the germ 

 is an extract jrom the history oj its ancestors, occasioned by the 

 laws of inheritance. 'X\\\i/>aliiigcnetic e>;iract appears essentially 

 disturbed only in case, through adaptation to the conditions of 

 embryonal life, cen^\^enetic changes have taken place. 



This phylogenetic interpretation of the ontogenetic phenomena 

 is, up to the present, the only explanation of the latter. But it 

 receives the most important confirmation and supplementation 

 from the results of comparative anatomy and palK jntology. It 

 is of course impossible to prove this by an exact method or 

 indeed an experiment, because all these biological disciplines 

 are, according to the nature of the matter, historical and philo- 

 sophical natural sciences. Their common object is the investi- 

 gation of historical events, which happened in the course ot 

 many millions of years, long before the appearance of the human 

 race on the surface of our youthful planet. The immediate and 

 mathematically exact conception of these events is therefore 

 altogether beyond the reach of possibility. 



Only by the critical consideration of the historical archives, by 

 a speculation which is just as circumspect as it is daring, an 

 approximate understanding here becomes indirectly possible. 

 Phy'ogeny uses these historical archives in the same manner and 

 according to the sime method as other historical disciplines do. 

 Just as the historian, by the help of chronicles, biographies, and 

 letters drav/s up a detailed representation of an event long past ; 

 as the archaeologist by the study of inscriptions, piece; of sculpture, 

 utensils, obtains the knowledge of the state of civilisation of a 

 race long extinct ; as the linguist by comparative investiga- 

 tion of all related living languages and their older written docu- 

 ments proves their development and origin from a common 

 ancestral language ; just in the same manner the naturalist of 

 to-day, by the critical use of the phylogenetic archives, of com- 

 parative anatomy, ontogeny, and paleontology, arrives at an 

 approximate understanding of the events which, in the course of 

 unmeasured periods, have caused the change of forms in the 

 organic life upon our globe. 



The history of the paiental forms of organism-, or phylo^eny, 

 can therefore be proved by an exact method or by experiment 

 just as little as this is the case with her older ,and more favoured 

 sister geology. But the high scientific value of the litter is never- 



