470 



NATURE 



[September ii, 1890 



ber of the group, the pearly nautilus, yet owing to the fact that 

 growth of the shell is effected by addition of shelly matter to 

 the part already present, and to the additions being made in 

 such manner that the older part of the shell persists unaltered, 

 it is possible, from examination of a single shell— and in the 

 case of fossils the shells are the only part of which we have 

 exact knowledge— to determine all the phases of its growth ; 

 just as in the shell of Orbitolites all the stages of development 

 are manifest on inspection of an adult specimen. 



In such a shell as nautilus or ammonites the central chamber 

 is the oldest or first formed one, to which the remaining chambers 

 are added in succession. If, therefore, the development of the 

 shell is a repetition of ancestral history, the central chamber 

 should represent the palseonlologically oldest form, and the re- 

 maining chambers in succession forms of more and more recent 

 origin. Ammonite shells present, more especially in their 

 sutures, and in the markings and sculpturing of their surface, 

 characters that are easily recognized, and readily preserved in 

 fossils ; and the group, consequently, is a very suitable one for 

 investigation from this standpoint. 



Wiirtenberger's admirable and well-known researches i have 

 shown that in the Ammonites such a correspondence between 

 historic and embryonic development does really exist ; that, for 

 example, in Aspidoceras the shape and markings of the shells in 

 young specimens differ greatly from those of adults, and that the 

 characters of the young shells are those of palseontologically 

 older forms. 



Another striking illustration of the correspondence between 

 the palseontological and developmental records is afforded by the 

 antlers of deer, in which the gradually increasing complication 

 of the antler in successive years agrees singularly closely with the 

 progressive increase in size and complexity shown by the fossil 

 series from the Miocene age to recent times. 



Of cases where a single specimen has sufficed to prove the 

 palseontological significance of a developmental character, 

 Archaeopteryx affords a typical example. In recent birds the 

 metacarpals are firmly fused with one another, and with the 

 distal series of carpals ; but in development the metacarpals are 

 at first, and for some time, distinct. In Archaopteryx this dis- 

 tinctness is retained in the adult, showing that what is now an 

 embryonic character in recent birds was formerly an adult one. 



Other examples might easily be quoted, but these will suffice 

 to show that the relation between paleontology and embryology, 

 first enunciated by Agassiz, and required by the recapitulation 

 theory, does in reality exist. There is much yet to be done in 

 this direction. A commencement, a most promising commence- 

 ment, has been made, but as yet only a few groups have been 

 seriously studied from this standpoint. 



It is a great misfortune that paleontology is not more gener- 

 rally and more seriously studied by men versed in embryology, 

 and that those who have so greatly advanced our knowledge of 

 the early development of animals should so seldom have tested 

 their conclusions as to the affinities of the groups they are con- 

 cerned with by direct reference to the ancestors themselves, as 

 known to us through their fossil remains. 



I cannot but feel that, for instance, the determination of the 

 affinities of fossil Mammalia, of which such an extraordinary 

 number and variety of forms are now known to us, would be 

 greatly facilitated by a thorough and exact knowledge of the 

 development, and especially the later development, of the skele- 

 ton in their existing descendants, and I regard it as a reproach that 

 such exact descriptions of the later stages of development should 

 not exist, even in the case of our commonest domestic animals. 



The pedigree of the horse has attracted great attention, and 

 has been worked at most assiduously, and we are now, largely 

 owing to the labours of American palaeontologists, able to refer 

 to a series of fossil forms commencing in the lowest Eocene 

 beds, and extending upwards to the most recent deposits, which 

 show a complete gradation from a more generalized mammalian 

 type to the highly sj^ecialized condition characteristic of the 

 horse and its allies, and which may reasonably be regarded as 

 indicating the actual line of descent of the horse. In this par- 

 ticular case, more frequently cited than any other, the evidence 

 is entirely palseontological. The actual development of the 

 horse has yet to be studied, and it is greatly to be desired that 

 it should be undertaken speedily. Klever's ^ recent work on the 



' "Studieniiber die Stammesgeschichte der Ammoniten. Ein geologis- 

 cher Beweis fiir die Darwin'sche Theorie " (Leipzig, 1880.) 



^ "Zur Kenntniss der Morphogenese des Kquidengeblsses," Morpho- 

 Jogisches yahrbuck, xv., 1889, p. 308. 



NO. 1089, VOL. 42] 



development of the teeth in the horse may be referred to as 

 showing that important and unexpected evidence is to be obtained 

 in this way. 



A brilliant exception to the statement just made as to the want 

 of exact knowledge of the later development of the more highly 

 organized animals is afforded by the splendid labours of Prof. 

 Kitchen Parker, whose recent death has deprived zoology of one 

 of her most earnest and single-minded students, and zoologists, 

 young and old alike, of a true and sincere friend. Prof. Parker's 

 extraordinarily minute and painstaking investigations into the 

 development of the vertebrate skull rank among the most re- 

 markable of zootomical achievements, and afford a rich mine of 

 carefully recorded facts, the full value and bearing of which we 

 are hardly yet able to appreciate. 



If further evidence as to the value and importance of the 

 recapitulation theory were needed, it would suffice to refer to 

 the influence which it has had on the classification of the animal 

 kingdom. Ascidians and Cirripedes may be quoted as im- 

 portant groups, the true affinities of which were first revealed by 

 embryology ; and in the case of parasitic animals the structural 

 modifications of the adult are often so great that but for the 

 evidence yielded by development their zoological position could 

 not be determined. It is now indeed generally recognized that 

 in doubtful cases embryology affords the safest of all clues, and 

 that the zoological position of such forms can hardly be regarded 

 as definitely established unless their development, as well as 

 their adult anatomy, is ascertained. 



It is owing to this recapitulation theory that embryology has 

 exercised so marked an influence on zoological speculation. 

 Thus the formation in most, if not in all, animals of the nervous 

 system and of the sense organs from the epidermal layer of the 

 skin, acquired a new significance when it was recognized that 

 this mode of development was to be regarded as a repetition of 

 the primitive mode of formation of such organs ; while the 

 vertebral theory of the skull affords a good example of a view, 

 once stoutly maintained, which received its death-blow through 

 the failure of embryology to supply the evidence requisite in its 

 behalf. The necessary limits of time and space forbid that I 

 should attempt to refer to even the more important of the 

 numerous recent discoveries in embryology, but mention may be 

 very properly made here of Sedgwick's determination of the 

 modeofdevelopmentof the body cavity in Peripatus, a discovery 

 which has thrown most welcome light on what was previously a 

 great morphological puzzle. 



We must now turn to another side of the question. Although it 

 is undoubtedly true that development is to be regarded as a re- 

 capitulation of ancestral phases, and that the embryonic history 

 of an animal presents to us a record of the race history, yet it is 

 also an undoubted fact, recognized by all writers on embryology, 

 that the record so obtained is neither a complete nor a straight- 

 forward one. 



It is indeed a history, but a history of which entire chapters 

 are lost, while in those that remain many pages are misplaced, 

 and others are so blurred as to be illegible ; words, sentences, 

 or entire paragraphs are omitted, and worse still, alterations or 

 spurious additions have been freely introduced by later hands, 

 and at times so cunningly as to defy detection. 



Very slight consideration will show that development cannot 

 in all cases be strictly a recapitulation of ancestral stages. It is 

 well known that closely allied animals may differ markedly in 

 their mode of development. The common frog is at first a tad- 

 pole, breathing by gills, a stage which is entirely omitted by the 

 West Indian Hylodes. A crayfish, a lobster, and a prawn are 

 allied animals, yet they leave the egg in totally different forms. 

 Some developmental stages, as the pupa condition of insects, 

 or the stage in the development of a dogfish in which the 

 oesophagus is imperforate, cannot possibly be ancestral stages. 

 Or again, a chick embryo of say the fourth day is clearly not an 

 animal capable of independent existence, and therefore cannot 

 correctly represent any ancestral condition, an objection which 

 applies to the developmental history of many, perhaps of most 

 animals. 



Haeckel long ago urged the necessity of distinguishing in 

 actual development between those characters which are really 

 historical and inherited, and those which are acquired or 

 spurious additions to the record. The former he termed palin- 

 genetic or ancestral characters, the latter cenogenetic or ac- 

 quired. The distinction is undoubtedly a true one, but an 

 I exceedingly difficult one to draw in practice. The causes which 



