l6 MANUAL OF ZOOLOGY. 



Protozoa, Ccelenterata, Annuloida, Annulosa, Mollusca, and 

 Vertebrata. We have, then, to remember that every member 

 of each of these primary divisions of the animal kingdom 

 agrees with every oth€r member of the same division in being 

 formed upon a certain definite plan or type of structure, and 

 differs from every other simply in the grade of its organisation, 

 or in other words, in the degree to which it exhibits specialisa- 

 tion of function. 



Von Baer's Law of Development. — ^As the study of living 

 beings in their adult condition shows us that the differences 

 between those which are constructed upon the same morpho- 

 logical type depend upon the degree to which specialisation 

 of function is carried, so the study . of development teaches 

 us that the changes undergone by any animal in passing from 

 the embryonic to the mature condition are due to the same 

 cause. All the members of any given sub-kingdom, when 

 examined in their earliest embryonic condition, are found to 

 present the same fundamental characters. As development 

 proceeds, however, they diverge from one another with greater 

 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 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, Cce- 

 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 peculi- 

 arities belonging to any other morphological type. Never- 

 theless, the differences which subsist between the members of 



