September ii, 1890] 



NATURE 



469 



the series of stages by which the cyclical or discoidal type of 

 shell was derived from the simpler spiral form. 



In Orhitolites tenuissinta, as Dr. Carpenter has shown,^ "the 

 whole transition is actually presented during the successive 

 stages of its growth. For it begins life as a Cornuspira, .... 

 its shell forming a continuous spiral tube, with slight interrup- 

 tions at the points at which its successive extensions commence ; 

 while its sarcodic body consists of a continuous coil with slight 

 constrictions at intervals. The second stage consists in the 

 opening out of its spire, and the division of its cavity at regular 

 intervals by transverse septa, traversed by separate pores, 

 exactly as in Peneroplis. The third stage is marked by the 

 subdivision of the ' peneropline ' chambers into chamberlets, 

 as in the early forms of Orbiculina. And the fourth consists in 

 the exchange of the spiral for the cyclical plan of growth, which 

 is characteristic of Orbitolites ; a circular disk of progressively 

 increasing diameter being formed by the addition of successive 

 annular zones around the entire periphery." 



The shells both of Foraminifera and of Mollusca afford 

 peculiarly instructive examples for the study of recapitulation. 

 As growth of the shell is effected by the addition of new shelly 

 matter to the part already existing, the older parts of the shell 

 are retained, often unaltered, in the adult ; and in favourable 

 cases, as in Orbitolites tenuissima, all the stages of development 

 can be determined by simple inspection of the adult shell. 



It is important to remember that the recapitulation theory, if 

 valid, must apply not merely in a general way to the develop- 

 ment of the animal body, but must hold good with regard to the 

 formation of each organ or system, and with regard to the later 

 equally with the earlier phases of development. 



Of individual organs, the brain of birds has been already 

 cited. The formation of the vertebrate liver as a diverticulum 

 from the alimentary canal, which is at first simple, but by the 

 folding of its walls becomes greatly complicated, is another good 

 example ; as is also the development of the vomer in amphibians 

 as a series of toothed plates, equivalent morphologically to the 

 placoid scales of fishes, which are at first separate, but later on 

 fuse together and lose the greater number of their teeth. 



Concerning recapitulation in the later phases of development 

 and in the adult animal, the mode of renewal of the nails or of 

 the epidermis generally is a good example, each cell commencing 

 its existence in an indifferent form in the deeper layers of the 

 I epidermis, and gradually acquiring the adult peculiarities as it 



approaches the surface, through removal of the cells lying 

 above it. 



The above examples, selected almost haphazard, will suffice 

 to illustrate the theory of recapitulation. 



The proof of the theory depends chiefly on its universal ap- 

 plicability to all animals, whether high or low in the zoological 

 scale, and to all their parts and organs. It derives, also, strong 

 support from the ready explanation which it gives of many 

 otherwise unintelligible points. 



Of these latter a familiar and most instructive instance is 

 afforded by rudimentary organs, i.e. structures which, like the 

 outer digits of the horse's leg, or the intrinsic muscles of the ear 

 of a man, are present in the adult in an incompletely developed 

 form, and in a condition in which they can be of no use to their 

 possessors ; or else structures which are present in the embryo, 

 but disappear completely before the adult condition is attained — 

 for example, the teeth of whalebone whales, or the branchial 

 clefts of all higher vertebrates. 



Natural selection explains the preservation of useful varia- 

 tions, but will not account for the formation and perpetuation 

 of useless organs ; and rudiments such as those mentioned 

 above would be unintelligible but for recapitulation, which 

 solves the problem at once, showing that these organs, though 

 now useless, must have been of functional value to the ancestors 

 of their present possessors, and that their appearance in the 

 ontogeny of existing forms is due to repetition of ancestral 

 characters. Such rudimentary organs are, as Darwin pointed 

 out, of larger relative or even absolute size in the embryo than 

 in the adult, because the embryo represents the stage in the 

 pedigree in which they were functionally active. 



Rudimentary organs are extremely common, especially 

 among the higher groups of animals, and their presence and 

 significance are now well understood. Man himself affords 

 numerous and excellent examples, not merely in his bodily 

 structure, but by his speech, dress, and customs. For the silent 



Phil. Trans., 1883, 



' "Oa an Abyssal Type of the Genus Orbitolii 

 Part ii. p. 553. 



NO. 1089. VOL. 42] 



letter b in the word doubt, or the w of answer, or the buttons 

 on his elastic side boots are as true examples of rudiments, un- 

 intelligible but for their past history, as are the ear muscles he 

 possesses but cannot use, or the gill-clefts, which are functional 

 in fishes and tadpoles, and are present, though useless, in the 

 embryos of all higher vertebrates, which in their early stages 

 the hare and the tortoise alike possess, and which are shared 

 with them by cats and by kings. 



Another consideration of the greatest importance arises from 

 the study of the fossil remains of the animals that formerly 

 inhabited the earth. It was the elder Agassiz who first 

 directed attention to the remarkable agreement between the em- 

 bryonic growth of animals and their palaeontological history. 

 He pointed out the resemblance between certain stages in the 

 growth of young fish and their fossil representatives, and 

 attempted to establish, with regard to fish, a correspondence 

 between their palseontological sequence and the successive stages 

 of embryonic development. He then extended his observations 

 to other groups, and stated his conclusions in these words : — ^ 

 " It may therefore be considered as a general fact, very likely to 

 be more fully illustrated as investigations cover a wider ground, 

 that the phases of development of all living animals correspond 

 to the order of succession of their extinct representatives in past 

 geological times." 



This point of view is of the utmost importance. If the de- 

 velopment of an animal is really a repetition of its ancestral 

 history, then it is clear that the agreement or parallelism which 

 Agassiz insists on between the embryological and palseontological 

 records must hold good. Owing to the attitude which Agassiz 

 subsequently adopted with regard to the theory of natural selec- 

 tion, there is some fear of his services in this respect failing to 

 receive full recognition, and it must not be forgotten that the 

 sentence I have quoted was written prior to the clear enunciation 

 of the recapitulation theory by Fritz Miiller. 



The imperfection of the geological record has been often 

 referred to and lamented. It is very true that our museums 

 afford us but fragmentary pictures of life in past ages ; that the 

 earliest volumes of the history are lost, and that of others but a 

 few torn pages remain to us ; but the later records are in far 

 more satisfactory condition. The actual number of specimens 

 accumulated from the more recent formations is prodigious ; 

 facilities for consulting them are far greater than they were ; the 

 international brotherhood of science is now fully established, 

 and the fault will be ours if the material and opportunities now 

 forthcoming are not rightly and fully utilized. 



By judicious selection of groups in which long series of speci- 

 mens can be obtained, and in which the hard skeletal parts, 

 which alone can be suitably preserved as fossils, afford reliable 

 indications of zoological affinity, it is possible to test directly 

 this correspondence between palaeontological and embryological 

 histories, while in some instances a single lucky specimen will 

 afford us, on a particular point, all the evidence we require. 



Great progress has already been made in this direction, and 

 the results obtained are of the most encouraging description. 



By Alexander Agassiz a detailed comparison was made be- 

 tween the fossil series and the developmental stages of recent 

 forms in the case of the Echinoids, a group peculiarly well 

 adapted for such an investigation. The two records agree re- 

 markably in many respects, more especially in the independent 

 evidence they give as to the origin of the asymmetrical forms 

 from more regular ancestors. The gradually increasing com- 

 plication in some of the historic series is found to be repeated 

 very closely in the development of their existing representatives ; 

 and with regard to the whole group, Agassiz concludes that,^ 

 "comparing the embryonic de\elopment with the palseonto- 

 logical one, we find a remarkable similarity in both, and in a 

 general way there seems to be a parallelism in the appearance 

 of the fossil genera and the successive stages of the developmenJ 

 of the Echini." 



Neumayr has followed similar lines, and by him, as by other 

 authorities on the group, there seems to be general agreement as 

 to the parallelism between the embryological and palaeonto- 

 logical records, not merely for Echini, but for other groups of 

 Echinodermata as well. 



The Tetrabranchiate Cephalopoda are an excellent group in 

 which to study the problem, for though no opportunity has yet 

 occurred for studying the embryology of the only surviving mem- 



' " Essay on Cla_ssification," 1859, p. 115. 



' " Palaeontological and Embryological Development." An Address 

 before the American Association for the Advancement of Science, 18B0. 



