TRANSACTIONS OF SECTION D. 827 
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 ege to the adult are no accidental freaks, no mere matters of 
developmental convenience, but represent more or less closely, in more or less 
modified manner, the successive ancestral stages through which the present con- 
dition 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 extraordinary attention this science has received, 
that I venture to address you this morning. 
A few illustrations from different 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 difficulties 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 com- 
pressed lateraliy; 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 organisation 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 employing it, z.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 any 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 differs from that of other vertebrates in 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 
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