22 MANUAL OF ZOOLOGY. 



less different, the range of possible modification being ap- 

 parently almost illimitable. The differences are due to the 

 different degrees of specialisation of function necessary to 

 perfect the adult; and therefore, as Von Baer put it, the pro- 

 gress of development is from the general to the special. 



It is upon a misconception of the true import of this law 

 that the theory arose, that every animal in its development 

 passed through a series of stages in which it resembles, in turn, 

 the different inferior members of the animal scale. With 

 regard to man, standing at the top of the whole animal 

 kingdom, this theory has been expressed as follows : "Human 

 organogenesis is a transitory comparative anatomy, 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 respectively to the permanent 

 types of the lower sub-kingdoms namely, the Protozoa, Ccelen- 

 terata, Echinodermata, Annulosa, and Mollusca before finally 

 assuming the true vertebrate 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 peculiari- 

 ties belonging to any other morphological type. Neverthe- 

 less, the differences which subsist between the members of 

 each sub-kingdom in their adult condition are truly referable 

 to the degree 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. To give a single example, the young 

 Gasteropod (amongst the Mollusca) transiently presents all the 

 essential characters which permanently distinguish the adult 

 Pteropod. The development of the Gasteropod, however, pro- 

 ceeds beyond this point, and the adult is much more highly 

 specialised than is the adult Pteropod. 



The theory of development held by the supporters of the 

 doctrine of Evolution is best expressed in the words of Prof. 

 Haeckel. According to this eminent naturalist, " Ontogenesis " 

 (or the development of the individual) " is the brief and rapid 

 recapitulation of phylogenesis" (or the development of the 

 species) "governed by the physiological functions of transmis- 

 sion (reproduction) and nutrition (adaptation). The organic 

 individual, during the rapid and brief course of its individual 



