I 6 MANUAL OF ZOOLOGY. 



great the apparent amount of diversity amongst them may be 

 arranged under no more than half-a-dozen primary morpho- 

 logical types or plans of structure. Upon one or other of 

 these five or six plans every known animal, whether living or 

 extinct, is constructed. It follows from the limited number of 

 primitive types or patterns, that great numbers of animals 

 must agree with one another in their morphological type'. It 

 follows also that all so agreeing can differ from one another 

 only in the sole remaining element of the question namely, 

 by the amount of specialisation of function which they exhibit. 

 Every animal, therefore, as Professor Huxley has well expressed 

 it, is the resultant of two tendencies, the one morphological, 

 the other physiological. 



The six types or plans of structure, upon one or other of 

 which all known animals have been constructed, are techni- 

 cally called " sub-kingdoms," and are known by the names 

 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 other 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 



