94 ELEMENTS OF BIOLOGY. 



animal kingdom, this theory has been expressed as follows : 

 — " Human organogenesis is a transitory comparative ana- 

 tomy, as, in its turn, comparative anatomy is a fixed and 

 permanent state of the organogenesis of man " (Serres). In 

 other words, the embryo of a Vertebrate animal was believed 

 to pass through a series of changes corresponding respect- 

 ively to the permanent types of the lower sub-kingdoms 

 — namely, the Protozoa, Ccelenterata, Annuloida, Annu- 

 losa, and Mollusca — before finally assuming the true ver- 

 tebrate characters. Such, however, is not truly the case. 

 The ovum of every animal is from the first impressed with 

 the power of developing in one direction only, and very 

 early exhibits the fundamental characters proper to its sub- 

 kingdom, never presenting the structural peculiarities be- 

 longing to any other morphological type. Nevertheless, the 

 differences which subsist between the members of each sub- 

 kingdom in their adult condition are truly referable to the 

 ^egree to which development proceeds, the place of each 

 individual in his own sub-kingdom being regulated by the 

 stage at which development is arrested. Thus, many cases 

 are known in which the younger stages of a given animal 

 represent the permanent adult condition of an animal some- 

 what lower in the scale. Thus, to give a single example, 

 the young of the water-breathing Univalve Shell-fish {Gas- 

 teropoda) transiently present all the essential characters 

 which distinguish the adult condition of the minute oceanic 

 Molluscs known as the Pteropods. The young Gasteropod, 

 namely, swims about freely by means of two lobes or fins 

 attached to the sides of the head (fig. 24, A), and similar 

 fins are present in the Pteropods in their adult condition 

 (fig. 24, B), enabling the animal to swim actively at the 

 surface of the open ocean. The development of the Gas- 

 teropod, however, proceeds beyond the point, and the adult 

 is much more highly specialised than is the adult Pteropod. 

 Upon the theory of " Evolution " such facts as the above 

 would be explained simply by the law of hereditary trans- 



