14 MANUAL OP ZOOLOGY. 



or less rapidity, until the adults ultimately become more or 

 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 Yon 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, Coe- 

 lenterata, Annuloida, 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 

 peculiarities belonging to any other morphological type. Never- 

 theless, 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. 



7. HOMOLOGY, ANALOGY, AND HOMOMOBPHISM. 



When organs in different animals agree with one another in 

 fundamental structure, they are said to be ' homologous ;' 

 when they perform the same functions they are said to be 

 1 analogous.' Thus the wing of a bird and the arm of a man 

 are constructed upon the same fundamental plan, and they 

 are, therefore, homologous organs. They are not analogous, 



