4 PROFESSOR A. M. MARSHALL. 



The Recapitulation Theory supplies a ready test. On employing 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 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 vetebrates 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 acquire \ one, for throughout all the earlier stages the 

 optic lobes are, as in other vetebrates, 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." Develop- 

 ment, 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 

 with spiral shells. Development 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 individual during its own 

 growth and development passes through the series of stages by which 

 the cyclical or discoidal type of shell was derived from the simpler 

 spiral form. 



In Orbitolites tenuissima, as Dr. Carpenter has shown, 1 ' the whole 



1 W. B. Carpenter, 'On an Abyssal Type of the Genns Orbitolites, ' Phil. 

 Trans. 1883, part ii. p. 553. 



