6 INTRODUCTION. 



ripening in their site to perfection, in resemblance of the parent, and 

 remaining as a permanent addition to the common stock. 



In as far as human observation extends, almost every living being 

 is propagated through a principle originating in the substance of the pa- 

 rent, whether as incorporated with its body, or preserved in a detached 

 form. Herein is the germ sustained by a pabulum generated along with 

 it under the same integument ; and it is nurtured until adapted for sepa- 

 ration, by growing strength and increasing dimensions. 



For the most part, both the ovum and the young are discharged 

 from the parent ; the former to bring its contents into existence, the 

 latter to reach that maturity essential for its well-being in an indepen- 

 dent state. 



But both the internal and the external ova of certain creatures are 

 ripened until ready to burst the integument, when they give birth to 

 the young while still included in the body of the parent, or affixed to 

 it ; and the young hydra remains attached to the parent, while some 

 still younger generate from its skin, or from that of its descendants, if 

 early separation do not ensue. 



No proper parallel can be drawn between the fertility of some 

 tribes, compared with the sterility of others. Thousands of ova, or a 

 multitude of young attain maturity, and are produced almost at the 

 same moment by fertile animals : in those which are less so, the progress 

 of the ovum is slow and gradual, occupying a long time, and if several, 

 only a single one is produced at once, after considerable intervals. 



Where confined in a transparent integument, nothing can be more 

 interesting to the contemplative than the progress of the embryo. At 

 first invisible, next the merest speck, then the development of origi- 

 nating parts, the current of the vital fluid carrying vigour to the ex- 

 tremities, and the motion of the members, all as life is awakened. 



The ovum of most of the larger animated tribes is invested by a 

 hard calcareous shell, sometimes capable of resisting great force and 

 pressure. In many of the smaller tribes it is readily crushed, and the 

 contents annihilated, being contained only in a flexible skin or capsule. 



Among the larger and more perfect creatures, their race is carried 



