September 



1890] 



NATURE 



479 



The advantage of cross-fertilization in increasing the vigour 

 of the offspring is well known, and in plants devices of the most 

 varied and evep extraordinary kind are adopted to ensure that 

 such cross-fertilization occurs. The essence of the act of 

 cross- fertilization, which is already established among Pro- 

 tozoa, consists in combination of the nuclei of two cells, 

 male and female, derived from different individuals. The 

 nature of the process is of such a kind that two individual 

 cells are alone concerned in it ; and it may, I think, be reason- 

 ably argued that the reason why animals commence their exist- 

 ence as eggs, i.e. as single cells, is because it is in this way only 

 that the advantage of cross-fertilization can be secured, an 

 advantage admittedly of the greatest importance, and to secure 

 which natural selection would operate powerfully. 



The occurrence of parthenogenesis, either occasionally or 

 normally, in certain groups is not, I think, a serious objection 

 to this view. There are very strong reasons for holding that 

 parthenogenetic development is a modified form, derived from 

 the sexual method. Moreover, the view advanced above does 

 not require that cross-fertilization should be essential to individual 

 development, but merely that it should be in the highest degree 

 advantageous to the species, and hence leaves room for the 

 occurrence, exceptionally, of parthenogenetic development. 



If it be objected that this is laying too much stress on sexual 

 reproduction, and on the advantage of cross- fertilization, then it 

 may be pointed out in reply that sexual reproduction is the 

 characteristic and essential mode of multiplication among 

 Metazoa : that it occurs in all Metazoa, and that when asexual 

 reproduction, as by budding, &c., occurs, this merely alternates 

 with the sexual process which, sooner or later, becomes 

 essential. 



If the fundamental importance of sexual reproduction to the 

 welfare of the species be granted, and if it be further admitted 

 that Metazoa are descended from Protozoa, then we see that 

 there is really a constraining force of a most powerful nature 

 compelling every animal to commence its life-history in the 

 unicellular condition, the only condition in which the advantage 

 of cross-fertilization can be obtained ; i.e. constraining every 

 animal to begin its development at its earliest ancestral stage, at 

 the very bottom of its genealogical tree. 



On this view the actual development of any animal is strictly 

 limited at both ends : it must commence as an egg, and it must 

 end in the likeness of the parent. The problem of recapitula- 

 tion becomes thereby greatly narrowed ; all that remains being 

 to explain why the intermediate stages in the actual develop- 

 ment should repeat the intermediate stages of the ancestral 

 history. 



Although narrowed in this way, the problem still remains one 

 of extreme difficulty. 



It is a consequence of the theory of natural selection that 

 identity of structure mvolves community of descent : a given 

 result can only be arrived at through a given sequence of events : 

 the same morphological goal canYiot be reached by two inde- 

 pendent paths. A negro and a white man have had common 

 ancestors in the past ; and it is through the long-continued action 

 of selection and environment that the two types have been 

 gradually evolved. You cannot turn a white man into a negro 

 merely by sending him to live in Africa : to create a negro the 

 whole ancestral history would have to be repeated ; and it may 

 be that it is for the same reason that the embryo must repeat or 

 recapitulate its ancestral history in order to reach the adult goal. 

 I am not sure that we can at present get much further ; but 

 the above considerations give opportunity for brief notice of 

 what is perhaps the most noteworthy of recent embryological 

 papers, Kleinenberg's remarkable monograph on Lopado- 

 rhynchus. 



Kleinenberg directs special attention to what is known to 

 evolutionists as the difficulty with regard to the origin of new 

 organs, which is to the effect that although natural selection is 

 competent to account for any amount of modification in an 

 organ after it has attained a certain size, and become of 

 functional importance, yet that it cannot account for the earlier 

 stages in the formation of an organ before it has become large 

 enough or sufficiently developed to be of real use. The. difficulty 

 is a serious one : it is carefully considered by Mr. Darwin, and 

 met completely in certain cases ; but, as Kleinenberg correctly 

 states, no general explanation has been offered with regard to 

 such instances. 



As such general explanation Kleinenberg proposes his theory 

 of the development of organs by substitution. He points out 



NO. 1089, VOL. 42] 



that any modification of an organ or tissue must involve 

 modification, at least in functional activity, of other organs. 

 He then continues by urging that one organ may replace or be 

 substituted for another, the replacing organ being in no way 

 derived morphologically from the replaced or preceding organ, 

 but having a genetic relation to it of this kind :— -that it can 

 only arise in an organism so constituted, and is dependent on 

 the prior existence of the replaced organ, which supplies the 

 necessary stimulus for its formation. 



As an example he takes the axial skeleton of vertebrates. 

 The notochord, formed by change of function from the wall of 

 the digestive canal, is the sole skeleton of the lowest vertebrates, 

 and the earliest developmental phase in all the higher forms. 

 The notochord gives rise directly to no other organ, but is 

 gradually replaced by other and unlike structures by substitu- 

 tion. The notochord is an intermediate organ, and the cartila- 

 ginous skeleton which replaces it is only intelligible through the 

 previous existence of the notochord ; while, in its turn, the 

 cartilaginous skeleton gives way, being replaced, through sub- 

 stitution, by the bony skeleton. 



The successive phases in the evolution of weapons might be 

 quoted as an illustration of Kleinenberg's theory. The bow and 

 arrow is a better weapon than a stick or stone ; it is used for 

 the same purpose, and the importance or need for a better 

 weapon led to the replacement of the sling by the bow ; the 

 bow does not arise by further development or increasing per- 

 fection of the sling ; it is an entirely new weapon, towards the 

 formation of which the older and more primitive weapons have 

 acted as a stimulus, and which has replaced these latter by sub- 

 stitution, while the substitution at a later date of firearms for 

 the bow and arrow is merely a further instance of the same 

 principle. 



It is too early yet to realize the full significance of Kleinen- 

 berg's most suggestive theory ; but if it be really true that each 

 historic stage in the evolution of an organ is necessary as a 

 stimulus to the development of the next succeeding stage, then 

 it becomes clear why animals are constrained to recapitulate. 

 Kleinenberg su2;gests further that the extraordinary persistence 

 in embryonic life of organs which are rudimentary and function- 

 less in the adult may also be explained by his theory, the 

 presence of such organs in the embryo being indispensable as a 

 stimulus to the development of the permanent structures of the 

 adult. 



It would be easy to point out difficulties in the way of the 

 theory. The omission of historic stages in the actual ontogenetic 

 development, of which almost all groups of animals supply 

 striking examples, is one of the most serious ; for if these stages 

 are necessary as stimuli for the succeeding stages, then their 

 omission requires explanation ; while, it such stimuli are not 

 necessary, the theory would appear to need revision. 



Such objections may, however, prove to be less serious than 

 they appear at first sight ; and in any case Kleinenberg's theory 

 may be welcomed as an important and original contribution, 

 which deserves — indeed demands — the fullest and most careful 

 consideration from all morphologists, and which acquires special 

 interest from the explanation which it offers of recapitulation as 

 a mechanical process, through which alone is it possible for an 

 embryo to attain the adult structure. 



That recapitulation does actually occur, that the several stages 

 in the development of an animal are inseparably linked with and 

 determined by its ancestral history, must ht. accepted. "To 

 take any other view is to admit that the structure of animals and 

 the history of their development form a mere snare to entrap our 

 judgment." 



Embryology, however, is not to be regarded as a master-key 

 that is to open the gates of knowledge and remove all obstacles 

 from our path without further trouble on our part ; it is rather to 

 be viewed and treated as a delicate and complicated instrument, 

 the proper handling of which requires the utmost nicety of 

 balance and adjustment, and which, unless employed with the 

 greatest skill and judgment, may yield false instead of true 

 results. 



Embryology is indeed a most powerful and efficient aid, but 

 it will not, and cannot, provide us with an immediate or com- 

 plete answer to the great riddle of life. Complications, dis- 

 tortions, innumerable and bewildering, confront us at every 

 step, and the progress of knowledge has so far served rather to 

 increase the number and magnitude of these pitfalls than to teach 

 us how to avoid them. 



