468 



NATURE 



[September ii, i8co 



matician in simplifying what has been already won as in securing 

 new conquests. I hope that the apathy of so many years may lead 

 to a splendid awakening in this country, and that our past 

 neglect of this most beautiful theory may be atoned for in the 

 future by special devotion and appreciation. 



SECTION D. 



Opening Address by Prof. A. Milnes Marshall, M.A., 

 M.C>., D.Sc, F.R.S., President of the Section. 



As my theme for this morning's address I have selected the 

 development of animals. I have made this choice from no 

 desire to extol one particular branch of biological study at the 

 expense of others, nor through failure to appreciate or at least 

 admire the work done and the results achieved in recent years 

 by those who are attacking the great problems of life from other 

 sides and with other weapons. 



My choice is determinerl by the necessity that is laid upon me, 

 through the wide range of sciences whose encouragement and 

 advancement are the peculiar privilege of this Section, to keep 

 within reasonable limits the direction and scope of my remarks ; 

 and is confirmed by the thought that, in addressing those 

 specially interested in and conversant with biological study, your 

 President acts wisely in selecting as the subject-matter of his 

 discourse some branch with which his own studies and inclina- 

 tions have brought him into close relation. 



Embryology, referred to by the greatest of naturalists as "one 

 of the most important subjects in the whole round of natural 

 history," is still in its youth, but has of late years thriven so 

 mightily that fear has been expressed lest it should absorb 

 unduly the attention of zoologists, or even check the progress 

 of science by diverting interest from other and equally important 

 branches. 



Nor is the reason of this phenomenal success hard to find. 

 The actual study of the processes of development ; the gradual 

 building up of the embryo, and then of the young animal, within 

 the egg ; the fashioning of its various parts and organs ; the 

 devices for supplying it with food, and for ensuring that the 

 respiratory and other interchanges are duly performed at all 

 stages — all these are matters of absorbing interest. Add to 

 these the extraordinary changes which may take place after 

 leaving the egg, the conversion, for instance, of the aquatic gill- 

 breathing tadpole — a true fish as regards all essential points of 

 its anatomy — into a four-legged frog, devoid of tail, and breath- 

 ing by lungs ; or the history of the metamorphosis by which the 

 sea-urchin is gradually buiit up within the body of its pelagic 

 larva, or the butterfly derived from its grub. Add to these 

 again the far wider interest aroused by comparing the life- 

 histories of allied animals, or by tracing the mode of develop- 

 ment of a complicated organ, e.g. the eye or the brain, in the 

 various animal groups, from its simplest commencement, through 

 gradually increasing grades of efficiency, up to its most perfect 

 form as seen in the highest animals. Consider this, and it 

 becomes easy to understand the fascination which embryology 

 exercises over those who study it. 



But all this is of trifling moment compared with the great 

 generalization which tells us that the development of animals 

 has a far higher meaning ; that the several embryological stages 

 and the order of their occurrence are no mere accidents, but are 

 forced on an animal in accordance with a law, the determination 

 of which ranks as one of the greatest achievements of biological 

 science. 



The doctrine of descent, or of evolution, teaches us that as 

 individual animals arise, not spontaneously, but by direct 

 descent from pre-existing animals, so also is it with species, with 

 families, and with larger groups of animals, and so also has it 

 been for all time ; that as the animals of succeeding generations 

 are related together, so also are those of successive geologic 

 periods ; that all animals, living or that have lived, are united 

 together by blood relationship of varying nearness or remoteness ; 

 and that every animal now in existence has a pedigree stretching 

 back, not merely for ten or a hundred generations, but through 

 all geologic time since the dawn of life on this globe. 



The study of development, in its turn, has revealed to us that 

 each animal bears the mark of its ancestry, and is compelled to 

 discover its parentage in its own development : that the phases 

 through which an animal passes in its progress from the egg to 



NO. 1089, VOL. 42] 



the adult are no accidental freaks, no mere matters of develop- 

 mental convenience, but represent more or less closely, in more 

 or less modified manner, the successive ancestral stages through 

 which the present condition has been acquired. 



Evolution tells us that each animal has had a pedigree in the 

 past. Embryology reveals to us this ancestry, because every 

 animal in its own development repeats this history, climbs up its 

 own genealogical tree. 



Such is the recapitulation theory, hinted at by Agassiz, and 

 suggested more directly in the writings of von Baer, but first 

 clearly enunciated by Fritz Miiller, and since elaborated by 

 many, notably by Balfour and by Ernst Haeckel. 



It is concerning this theory, which forms the basis of the 

 science of embryology, and which alone justifies the extra- 

 ordinary attention this science has received, that I venture to 

 address you this morning. 



A few illustrations from diff'erent groups of animals will best 

 explain the practical bearings of the theory, and the aid which 

 it affords to the zoologist of to-day ; while these will also serve 

 to illustrate certain of the difticulties which have arisen in the 

 attempt to interpret individual development by the light of past 

 history — difficulties which I propose to consider at greater 

 length. 



A very simple example of recapitulation is afforded by the 

 eyes of the sole, plaice, turbot, and their allies. These "flat 

 fish " have their bodies greatly compressed laterally ; and the 

 two surfaces, really the right and left sides of the animal, unlike, 

 one being white, or nearly so, and the other coloured. The 

 flat fish has two eyes, but these, in place of being situated, as in 

 other fish, one on each side of the head, are both on the coloured 

 side. The advantage to the fish is clear, for the natural position 

 of rest of a flat fish is lying on the sea bottom, with the white 

 surface downwards and the coloured one upwards. In such a 

 position an eye situated on the white surface could be of no use 

 to the fish, and might even become a source of danger, owing 

 to its liability to injury from stones or other hard bodies on the 

 sea bottom. 



No one would maintain that flat fish were specially created as 

 such. The totality of their organization shows clearly enough 

 that they are true fish, akin to others in which the eyes are 

 symmetrically placed one on each side of the head, in the 

 position they normally hold among vertebrates. We must 

 therefore suppose that flat fish are descended from other fish in 

 which the eyes are normally situated. 



The recapitulation theory supplies a ready test. On employ- 

 ing it, i.e. on studying the development of the flat fish, we 

 obtain a conclusive answer. The young sole on leaving the egg 

 is shaped just as an ordinary fish, and has the two eyes placed 

 symmetrically on the two sides of the head. It is only after 

 the young fish has reached some size, and has begun to approach 

 the adult in shape, and to adopt its habit of resting on one side 

 on the sea bottom, that the eye of the side on which it rests 

 becomes shifted forwards, then rotated on to the top of the 

 head, and finally twisted completely over to the opposite side. 



The brain of a bird diff"ers from that of other vertebrates irk 

 the position of the optic lobes, these being situated at the sides 

 instead of on the dorsal surface. Development shows that this 

 lateral position is a secondarily acquired one, for throughout all 

 the earlier stages the optic lobes are, as in other vertebrates, on 

 the dorsal surface, and only shift down to the sides shortly 

 before the time of hatching. 



Crabs differ markedly from their allies, the lobsters, in the 

 small size and rudimentary condition of their abdomen or 

 " tail." Development, however, affords abundant evidence of 

 the descent of crabs from macrurous ancestors, for a young crab- 

 at what is termed the Megalopa stage has the abdomen as large 

 as a lobster or prawn at the same stage. 



Molluscs afford excellent illustrations of recapitulation. The 

 typical gastropod has a large spirally-coiled shell ; the limpet,, 

 however, has a large conical shell, which in the adult gives no 

 sign of spiral twisting, although the structure of the animal 

 shows clearly its affinity to forms wtth spiral shells. Develop- 

 ment solves the riddle at once, telling us that in its early stages 

 the limpet embryo has a spiral shell, which is lost on the 

 formation, subsequently, of the conical shell of the adult. 



Recapitulation is not confined to the higher groups of animals, 

 and the Protozoa themselves yield most instructive examples. 

 A very striking case is that of Orbitolites, one of the most 

 complex of the porcellanous Foraminifera, in which each indi- 

 vidual during its own growth and development passes through 



