MODES OF REPRODUCTION 317 



escape by the posterior. The dorsal and ventral walls of the 

 capsule are united in the new-laid egg by a membrane, which 

 gradually weakens until at hatching it gives way, and the an- 

 terior lips of the capsule spring apart, and leave an opening 

 through which the young fish makes its exit. Professor Bash- 

 ford Dean has argued that this adaptation of the capsule in 

 several minute particulars to the needs of an embryo which 

 will not exist till long after the capsule is formed is evidence of 

 evolution in a determinate direction, and cannot be explained 

 either on Lamarckian principles, that is by the direct action of 

 conditions, or on the principle of selection. The embryo could 

 not affect the shape or structure of the capsule directly, since 

 the capsule is formed first, and the young fish which was 

 selected by the conditions of its life would not necessarily be 

 the one which had a spontaneous variation to the right kind of 

 capsule. There are, however, considerations which Professor 

 Dean has overlooked. He makes too much of the adaptation 

 in shape, since in other cases, as in the bird and the Teleostean 

 fish, we know that the egg-shell is not in the least adapted to 

 the shape of the embryo, which is packed away inside it in the 

 most complicated distortions. Then again it is possible that 

 the shape of the capsule may have determined to some extent 

 the shape of the fish by its effect on the embryo, as for instance 

 on the tail, supposing that the tail was of little importance to 

 the free life of the fish after hatching, so that it might retain the 

 shape to which it was forced by the egg-shell. On the other 

 hand, the elongated body of the young snake, and the broad flat 

 fins of the embryo of a skate or ray, have to be coiled up in an 

 egg-shell whose shape shows no correspondence whatever with 

 that of the young animal which is hatched from it. It is 

 illogical, therefore, to conclude that the egg-shell of the Chimae- 

 roid alone is precisely adapted to the shape of the animal 

 subsequently developed in it, and then to inquire how this 

 adaptation was evolved. It is more reasonable to assume that 

 in all cases the shape of the egg-shell is determined, as in birds 

 and reptiles, by the mechanical and physiological conditions 

 existing in the oviduct, and has no relation to the form of the 

 embryo. In fact the prolongation of the shell of Chimaera 

 into a narrow tube with flat flanges along its edges can be ex- 

 plained satisfactorily by the manner in which the egg is laid. 



